[clang] [clang] use relative paths for builtin headers during module compilation (PR #68023)

[Richard Howell via cfe-commits]

rmaz wrote:

How about the updated approach? This will use a different include dir based on if the header is a builtin header name or not, which also solves the issue of having to pass the extra include path to the frontend.

https://github.com/llvm/llvm-project/pull/68023