[PATCH 1/4] [clang] Improve nested name specifier AST representation

This is a major change on how we represent nested name qualifications in the AST.

* The nested name specifier itself and how it's stored is changed. The prefixes for types are handled within the type hierarchy, which makes canonicalization for them super cheap, no memory allocation required. Also translating a type into nested name specifier form becomes a no-op. An identifier is stored as a DependentNameType. The nested name specifier gains a lightweight handle class, to be used instead of passing around pointers, which is similar to what is implemented for TemplateName. There is still one free bit available, and this handle can be used within a PointerUnion and PointerIntPair, which should keep bit-packing aficionados happy.
* The ElaboratedType node is removed, all type nodes in which it could previously apply to can now store the elaborated keyword and name qualifier, tail allocating when present.
* TagTypes can now point to the exact declaration found when producing these, as opposed to the previous situation of there only existing one TagType per entity. This increases the amount of type sugar retained, and can have several applications, for example in tracking module ownership, and other tools which care about source file origins, such as IWYU. These TagTypes are lazily allocated, in order to limit the increase in AST size.

This patch offers a great performance benefit.

It greatly improves compilation time for [stdexec](https://github.com/NVIDIA/stdexec). For one datapoint, for `test_on2.cpp` in that project, which is the slowest compiling test, this patch improves `-c` compilation time by about 7.2%, with the `-fsyntax-only` improvement being at ~12%.

This has great results on compile-time-tracker as well:
![image](https://github.com/user-attachments/assets/700dce98-2cab-4aa8-97d1-b038c0bee831)

This patch also further enables other optimziations in the future, and will reduce the performance impact of template specialization resugaring when that lands.

It has some other miscelaneous drive-by fixes.

About the review: Yes the patch is huge, sorry about that. Part of the reason is that I started by the nested name specifier part, before the ElaboratedType part, but that had a huge performance downside, as ElaboratedType is a big performance hog. I didn't have the steam to go back and change the patch after the fact.

There is also a lot of internal API changes, and it made sense to remove ElaboratedType in one go, versus removing it from one type at a time, as that would present much more churn to the users. Also, the nested name specifier having a different API avoids missing changes related to how prefixes work now, which could make existing code compile but not work.

How to review: The important changes are all in `clang/include/clang/AST` and `clang/lib/AST`, with also important changes in `clang/lib/Sema/TreeTransform.h`.

The rest and bulk of the changes are mostly consequences of the changes in API.

PS: TagType::getDecl is renamed to `getOriginalDecl` in this patch, just for easier to rebasing. I plan to rename it back after this lands.

Fixes #136624
Fixes #147000

Author: mizvekov

clang/docs/ReleaseNotes.rst

@@ -84,6 +84,7 @@ ABI Changes in This Version
 AST Dumping Potentially Breaking Changes
 ----------------------------------------
 
+- How nested name specifiers are dumped and printed changes, keeping track of clang AST changes.
 - Added support for dumping template arguments of structural value kinds.
 
 Clang Frontend Potentially Breaking Changes
@@ -481,6 +482,7 @@ Improvements to Clang's diagnostics
   under the subgroup ``-Wunsafe-buffer-usage-in-libc-call``.
 - Diagnostics on chained comparisons (``a < b < c``) are now an error by default. This can be disabled with
   ``-Wno-error=parentheses``.
+- Fix duplicate diagnostics for incomplete type in nested name specifier. (#GH147000)
 - Similarly, fold expressions over a comparison operator are now an error by default.
 - Clang now better preserves the sugared types of pointers to member.
 - Clang now better preserves the presence of the template keyword with dependent
@@ -662,7 +664,7 @@ Improvements to Clang's diagnostics
   #GH142457, #GH139913, #GH138850, #GH137867, #GH137860, #GH107840, #GH93308,
   #GH69470, #GH59391, #GH58172, #GH46215, #GH45915, #GH45891, #GH44490,
   #GH36703, #GH32903, #GH23312, #GH69874.
-  
+
 - Clang no longer emits a spurious -Wdangling-gsl warning in C++23 when
   iterating over an element of a temporary container in a range-based
   for loop.(#GH109793, #GH145164)
@@ -950,8 +952,11 @@ Bug Fixes to AST Handling
 - Fixed a malformed printout of ``CXXParenListInitExpr`` in certain contexts.
 - Fixed a malformed printout of certain calling convention function attributes. (#GH143160)
 - Fixed dependency calculation for TypedefTypes (#GH89774)
+- Fix incorrect name qualifiers applied to alias CTAD. (#GH136624)
 - The ODR checker now correctly hashes the names of conversion operators. (#GH143152)
 - Fixed the right parenthesis source location of ``CXXTemporaryObjectExpr``. (#GH143711)
+- Fixed ElaboratedTypes appearing within NestedNameSpecifier, which was not a
+  legal representation. This is fixed because ElaboratedTypes don't exist anymore. (#GH43179) (#GH68670) (#GH92757)
 - Fixed a crash when performing an ``IgnoreUnlessSpelledInSource`` traversal of ASTs containing ``catch(...)`` statements. (#GH146103)
 
 Miscellaneous Bug Fixes
@@ -1103,6 +1108,8 @@ AST Matchers
 ------------
 
 - Ensure ``isDerivedFrom`` matches the correct base in case more than one alias exists.
+- Removed elaboratedType matchers, and related nested name specifier changes,
+  following the corresponding changes in the clang AST.
 - Extend ``templateArgumentCountIs`` to support function and variable template
   specialization.
 

clang/include/clang/AST/ASTConcept.h

@@ -15,7 +15,7 @@
 #define LLVM_CLANG_AST_ASTCONCEPT_H
 
 #include "clang/AST/DeclarationName.h"
-#include "clang/AST/NestedNameSpecifier.h"
+#include "clang/AST/NestedNameSpecifierBase.h"
 #include "clang/AST/TemplateBase.h"
 #include "clang/Basic/SourceLocation.h"
 #include "clang/Basic/UnsignedOrNone.h"
@@ -175,12 +175,7 @@ class ConceptReference {
 
   SourceLocation getLocation() const { return getConceptNameLoc(); }
 
-  SourceLocation getBeginLoc() const LLVM_READONLY {
-    // Note that if the qualifier is null the template KW must also be null.
-    if (auto QualifierLoc = getNestedNameSpecifierLoc())
-      return QualifierLoc.getBeginLoc();
-    return getConceptNameInfo().getBeginLoc();
-  }
+  SourceLocation getBeginLoc() const LLVM_READONLY;
 
   SourceLocation getEndLoc() const LLVM_READONLY {
     return getTemplateArgsAsWritten() &&