[clang] [llvm][Stmt]:Fix musttail attribute on a function with not_tail_called attribute has no warning/error (PR #134465)

[Erich Keane via cfe-commits]

https://github.com/erichkeane requested changes to this pull request.

First:
The title of this PR has `llvm` and `stmt` in it, neither of which should be used here, so don't bother with a tag.

Second:
The PR message (which we use as a commit message) needs a ton more detail.

Third:
Please add a release note to this.

Four:
This needs tests to show that what it is doing is correct.

Five: 
I don't think this is really a good idea to make this an error, not a warning under `IgnoredAttributes`.  It seems more reasonable to just warn here.

Six: 
The diagnostic I think needs rewording.  It isn't clear that it is referring to either the `musttail` attribute nor the `not_tail_called` attribute.

https://github.com/llvm/llvm-project/pull/134465