[clang] [llvm] [Clang] Make -ffile-compilation-dir= support reproducible sanitizer builds (PR #201803)

[Björn Svensson via cfe-commits]

bjosv wrote:

> Question regarding this approach versus #186908: if one already has a build using `-ffile-prefix-map` but is hitting reproducible build issues only when sanitizers are enabled, would the recommendation be to apply `-fsanitize-compilation-dir=` _in addition to_ `-ffile-prefix-map`?
> 

After some review comments this PR only provides `-ffile-compilation-dir=` to the user, and using that together with existing `-ffile-prefix-map` should be fine.
I believe both needs to be used to also sanitize `__FILE__` macros, e.g.`-ffile-compilation-dir=. -fmacro-prefix-map=$(PWD)=.`

https://github.com/llvm/llvm-project/pull/201803