[clang] [clang] Add a CodeGen option to ignore compilation directories (PR #149897)

[Steven Wu via cfe-commits]

cachemeifyoucan wrote:

> & then for now, in your use case, you'd be using the cc1 flag via -Xclang, etc?

Our explicit module build comes from dependency scanner, where it can generate the cc1 arguments we want to achieve empty compilation directory if needed. 

https://github.com/llvm/llvm-project/pull/149897