[clang] [Clang][Headers] Workaround for UCRT's `<complex.h>` in `<tgmath.h>` (PR #121650)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Is there a way to add test coverage for the changes? Also, this should come with a release note so users know about the fix.

https://github.com/llvm/llvm-project/pull/121650