[clang] [clang][modules] Allow module maps with textual headers to be non-affecting (PR #89441)

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

> I've sent a version of this as #89729, based on your first commit here.

I left a comment there.

> > I think one option we have here is to consider all module maps describing a textual header that got included as affecting. I'm concerned that a long chain of textual header includes might again be problematic.
> 
> Yeah, that's the option that comes closest to preserving previous behavior, and the one that I think most precisely captures the "affecting" semantics - including the module maps that were required for the compilation.

Ok, if that doesn't explode your builds again, it makes sense to consider those module maps affecting as well. I updated the patch that now checks for `Preprocessor::alreadyIncluded()` for textual headers.

https://github.com/llvm/llvm-project/pull/89441