[clang] [ASTWriter] Do not allocate source location space for module maps used only for textual headers (PR #116374)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

Finally got to it, there is already a test that checks this, see:
https://github.com/llvm/llvm-project/blob/ef86a31a593a023946a2dc805948360efdad3930/clang/test/Modules/preprocess-decluse.cpp#L1

and it passes successfully.

I've confirmed why it works too. The module map file gets reread [here](https://github.com/llvm/llvm-project/blob/ef86a31a593a023946a2dc805948360efdad3930/clang/lib/Frontend/FrontendAction.cpp#L656), this function still correctly reports those module map files.

@jansvoboda11 please let me know if you have any other concerns.

https://github.com/llvm/llvm-project/pull/116374