[clang] [Clang][Preprocessor] Unify header-name lookahead for import and include (PR #191004)

[via cfe-commits]

yronglin wrote:

> We can revisit if someone hits this in practice and wants an extension.
> No platform-related concerns that we know of from the z/OS side.

I can create an RFC in the community if needed.


https://github.com/llvm/llvm-project/pull/191004