[llvm] [DiagnosticInfo] Output full file path instead of relative path (PR #68027)

[Paul Kirth via llvm-commits]

ilovepi wrote:

@aeubanks  You can control debug info paths being relative/absolute w/ compiler options. `-fdebug-prefix-map` and `-no-canonical-prefixes` come to mind. There are probably others too, but I haven't looked in a long time, and its not a set of options I've needed to deal with all that much. 

I believe those options have an effect on remarks metadata too, but I haven't checked.

@CongzheUalberta I think what you're after is consistency, so if those options control whether we use relative paths or not, perhaps we can just use whatever codegen flags are present to make both the preLTO and LTO piplelines use the same convention? Maybe a helper/support API can be set appropriately to simplify things.

I'm not sure what the behavior should be if the user sets the flags differently between the two  compiler invocations (e.g. for prelink and LTO pipelines).

https://github.com/llvm/llvm-project/pull/68027