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

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

> @jansvoboda11 still good with this change, or is there a less invasive way to get this done?

Hi, sorry for being unresponsive, I'm out of office until the end of the week. This looks reasonable to me, but I'd like to run a smoke test with the Darwin SDK first. I can get back to you early next week. Sounds good?

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