[clang] [clang modules] Setting `DebugCompilationDir` when it is safe to ignore current working directory (PR #128446)

[Jan Svoboda via cfe-commits]

https://github.com/jansvoboda11 requested changes to this pull request.

Leaving the correctness of the change up for other reviewers. My only gripe is the `const_cast`.

https://github.com/llvm/llvm-project/pull/128446