[clang] Remove duplicate API (PR #132776)

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll approved this pull request.

I talked to Aaron offline whether there's a better way to mark duplicates as deprecated, but given the variety of C compilers that can consume this header, we don't think there's a better way than a doxygen comment

https://github.com/llvm/llvm-project/pull/132776