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

[Sam McCall via cfe-commits]

sam-mccall wrote:

> Hmm, this will probably only work if you compile `A` into a PCM and then pass it to `B`. This is not how "Modules/preprocess-decluse.cpp" is set up.

In our case we always have a chain A <- B <- C.
A.modulemap can be affecting for B but should not be for C.
(Approximately, A covers approximately clang's headers (textual), B covers the standard library (modular). Every other library C sees A.modulemap, B.modulemap, B.pcm. There is no PCM for A).

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