[clang] [AST] Include clang/Basic/DiagnosticComment.h instead of clang/AST/CommentDiagnostic.h (PR #117499)

[Richard Trieu via cfe-commits]

Weverything wrote:

I support getting rid of the Diagnostic.h files outside of the Basic directory, at least when they are just forwarding headers.  From a layering point of view, Clang_Diagnostics should be the same level as Basic so it shouldn't be having things from AST.  Removing the forwarding headers will help clean up the layering.

https://github.com/llvm/llvm-project/pull/117499