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

[via cfe-commits]

yronglin wrote:

> I am not sure if we should apply the same policy between the `import` and the `include` cases.
> 
> For the `include` case, if the program is ill-formed under https://cplusplus.github.io/CWG/issues/3078.html, it may be reasonable to accept the program as an extension (with an appropriate diagnostic).

Hmm, I'm not sure too. Should we allow these rather "hacky" `header name`  and treat these as clang extensions? 
Do you have any suggestions @zygoloid @cor3ntin 


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