[clang] [Clang][Sema] Reject declaring an alias template with the same name as its template parameter. (PR #123533)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

when the discussion is eventually resolved please add a more detailed summary explaining how the fix is implemented and why.

These go in the git log and it is important for relevant details to show up there w/o the need for git show etc

https://github.com/llvm/llvm-project/pull/123533