[Clang][AST] Fix a crash on attaching doc comments (#78716)

This crash is basically caused by calling
`ASTContext::getRawCommentForDeclNoCacheImp` with its input arguments
`RepresentativeLocForDecl` and `CommentsInTheFile` refering to different
files. A reduced reproducer is provided in this patch.

After the source locations for instantiations of funtion template are
corrected in the commit 256a0b2, the
variable `CommitsInThisFile` in the function
`ASTContext::attachCommentsToJustParsedDecls` would refer to the source
file rather than the header file for implicit function template
instantiation. Therefore, in the first loop in
`ASTContext::attachCommentsToJustParsedDecls`, `D` should also be
adjusted for relevant scenarios like the second loop.

Fixes #67979 
Fixes #68524
Fixes #70550

clang/lib/AST/ASTContext.cpp

@@ -498,7 +498,11 @@ void ASTContext::attachCommentsToJustParsedDecls(ArrayRef<Decl *> Decls,
     return;
 
   FileID File;
-  for (Decl *D : Decls) {
+  for (const Decl *D : Decls) {
+    if (D->isInvalidDecl())
+      continue;
+
+    D = &adjustDeclToTemplate(*D);
     SourceLocation Loc = D->getLocation();
     if (Loc.isValid()) {
       // See if there are any new comments that are not attached to a decl.

clang/test/AST/ast-crash-doc-function-template.cpp

@@ -0,0 +1,30 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -x c++ -Wdocumentation -fsyntax-only -ast-dump-all %t/t.cpp
+
+//--- t.h
+/// MyClass in the header file
+class MyClass {
+public:
+  template <typename T>
+  void Foo() const;
+
+  /// Bar
+  void Bar() const;
+};
+
+//--- t.cpp
+#include "t.h"
+
+/// MyClass::Bar: Foo<int>() is implicitly instantiated and called here.
+void MyClass::Bar() const {
+  Foo<int>();
+}
+
+/// MyClass::Foo
+template <typename T>
+void MyClass::Foo() const {
+}
+
+// CHECK: TranslationUnitDecl