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

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> @hubert-reinterpretcast Does z/OS have something like that? I think that'd be the most likely place.

To my knowledge (and I also asked some other people), there is no particular reason why header resource identifiers would begin with a `%`, `:`, `=`, or `<` on z/OS more commonly than on other platforms.

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