[demangler] Fix demangling a template argument which happens to be a null pointer

As seen in llvm/llvm-project#51854
llvm-cxxfilt was having trouble demangling the case "_Z1fIDnLDn0EEvv".
We handled the "LDNE" case and "LPi0E" but not "LDn0E". This change adds
that handling.

Differential Revision: https://reviews.llvm.org/D124010

Author: gbreynoo

libcxxabi/src/demangle/ItaniumDemangle.h

@@ -4340,7 +4340,7 @@ Node *AbstractManglingParser<Derived, Alloc>::parseExprPrimary() {
     return nullptr;
   }
   case 'D':
-    if (consumeIf("DnE"))
+    if (consumeIf("Dn") && (consumeIf('0'), consumeIf('E')))
       return make<NameType>("nullptr");
     return nullptr;
   case 'T':

libcxxabi/test/test_demangle.pass.cpp

@@ -644,6 +644,7 @@ const char* cases[][2] =
     {"_ZNSs4_Rep20_S_empty_rep_storageE", "std::string::_Rep::_S_empty_rep_storage"},
     {"_ZZN23ImageLoaderMachOClassic26bindIndirectSymbolPointersERKN11ImageLoader11LinkContextEbbE13alreadyWarned", "ImageLoaderMachOClassic::bindIndirectSymbolPointers(ImageLoader::LinkContext const&, bool, bool)::alreadyWarned"},
     {"_ZN12_GLOBAL__N_115emergency_mutexE", "(anonymous namespace)::emergency_mutex"},
+    {"_Z1fIDnLDn0EEvv", "void f<std::nullptr_t, nullptr>()"},
 
     // clang
 

llvm/include/llvm/Demangle/ItaniumDemangle.h

@@ -4340,7 +4340,7 @@ Node *AbstractManglingParser<Derived, Alloc>::parseExprPrimary() {
     return nullptr;
   }
   case 'D':
-    if (consumeIf("DnE"))
+    if (consumeIf("Dn") && (consumeIf('0'), consumeIf('E')))
       return make<NameType>("nullptr");
     return nullptr;
   case 'T':