[clang] [Clang] Warn on deprecated specializations used in system headers. (PR #70353)

[Louis Dionne via cfe-commits]

ldionne wrote:

@cor3ntin This is really nice, thanks for the patch! The CI should pass if you merge `main` into your branch -- you were the victim of a temporary CI issue that has been fixed since then.

https://github.com/llvm/llvm-project/pull/70353