[clang] [clang][lex] Introduce new single-module-parse mode (PR #135813)

[Michael Spencer via cfe-commits]

https://github.com/Bigcheese approved this pull request.

I think this looks good overall. I'm a little concerned that the single file and single module names are similar while having quite different semantics, but single file is never actually exposed to users, so they won't be confused. I don't have any better suggestions for naming, but if you come up with one feel free to rename it (or rename single file).

https://github.com/llvm/llvm-project/pull/135813