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

[Sam McCall via cfe-commits]

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+// This test checks that a module map with a textual header can be marked as
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sam-mccall wrote:

This is a useful test, I think there are a couple of other affecting-ness properties that are important to test:

 - that a textual header that *is* included means its module is affecting (i.e. the include matters)
 - that this textual-affecting-ness doesn't leak outside the module (a module depending on B isn't affected by A.modulemap)

I tried to cover these in https://github.com/llvm/llvm-project/pull/89729/files#diff-2eed05f9a85bc5a79b9651ee0f23e5f1494e94a2f32e093847aa6dae5ce5d839 - it might be nice to add these as a second test here (I can do it as a followup if you prefer)

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