[clang] [clang] emit an error when the same identifier appears with both internal and external linkage in a translation unit (PR #78064)

[via cfe-commits]

cor3ntin wrote:

This change needs a release note.
Please add an entry to `clang/docs/ReleaseNotes.rst` in the section the most adapted to the change (Bug fixes), and referencing any Github issue this change fixes. Thanks!

https://github.com/llvm/llvm-project/pull/78064