Driver: Avoid llvm::sys::path::append if resource directory absolute.

After #145996 CLANG_RESOURCE_DIR can be an absolute path so we need to
handle it correctly in the driver.

llvm::sys::path::append does not append absolute paths in the way
that I expected (or consistent with other similar APIs such as C++17
std::filesystem::path::append or Python os.path.join); instead, it
effectively discards the leading / and appends the resulting relative path
(e.g. append(P, "/bar") with P = "/foo" sets P to "/foo/bar").

Many tests start failing if I try to align llvm::sys::path::append with
the other APIs because of callers that expect the existing behavior,
so for now let's add a special case here for absolute resource paths,
and document the behavior in Path.h.

Reviewers: MaskRay

Reviewed By: MaskRay

Pull Request: #146449

Author: pcc

clang/lib/Driver/Driver.cpp

@@ -192,7 +192,12 @@ std::string Driver::GetResourcesPath(StringRef BinaryPath) {
 
   StringRef ConfiguredResourceDir(CLANG_RESOURCE_DIR);
   if (!ConfiguredResourceDir.empty()) {
-    llvm::sys::path::append(P, ConfiguredResourceDir);
+    // FIXME: We should fix the behavior of llvm::sys::path::append so we don't
+    // need to check for absolute paths here.
+    if (llvm::sys::path::is_absolute(ConfiguredResourceDir))
+      P = ConfiguredResourceDir;
+    else
+      llvm::sys::path::append(P, ConfiguredResourceDir);
   } else {
     // On Windows, libclang.dll is in bin/.
     // On non-Windows, libclang.so/.dylib is in lib/.

llvm/include/llvm/Support/Path.h

@@ -223,6 +223,8 @@ LLVM_ABI bool remove_dots(SmallVectorImpl<char> &path,
 ///   /foo  + bar/f => /foo/bar/f
 ///   /foo/ + bar/f => /foo/bar/f
 ///   foo   + bar/f => foo/bar/f
+///   foo   + /bar/f => foo/bar/f (FIXME: will be changed to /bar/f to align
+///                                with C++17 std::filesystem::path::append)
 /// @endcode
 ///
 /// @param path Set to \a path + \a component.