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

[Sam McCall via cfe-commits]

https://github.com/sam-mccall approved this pull request.

Thanks! This looks good, much neater than my approach.

I'm interested in whether the DirectUses change is related to this, and would like to slap on a couple of tests.

But either way, this looks good and it would be great to have it landed!

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