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

[Boaz Brickner via cfe-commits]

bricknerb wrote:

> Added @bricknerb to help with reviewing this (esp. the code style aspects)

Focused on code style, please let me know if you want me to dive in more to correctness.

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