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

[Jan Svoboda via cfe-commits]

jansvoboda11 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.

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