[clang] [Clang] Diagnose UB and emit error when identifier has both internal and external linkage (PR #192116)

[Aditya Medhane via cfe-commits]

flash1729 wrote:

hey @AaronBallman, addressed all your review points.
@cor3ntin added a C++ test with `expected-no-diagnostics` and a FIXME pointing at CWG 426.
Currently i am reading more into the C++ part and will like to address that in follow up as suggested by Aaron.

https://github.com/llvm/llvm-project/pull/192116