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

[via llvm-commits]

CongzheUalberta wrote:

> @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).

Thanks. I've tried `-fdebug-prefix-map` and `-no-canonical-prefixes` but still the preLTO and LTO behaviors seem inconsistent. I'm wondering if there exists some sort of codegen flags that make both the preLTO and LTO piplelines use the same convention? IIUC for DiagonsticInfo each component (clang, opt, llvm-dis, etc) has its own independent implementation of `DiagnosticHandler` with its own behavior, hence I'm not sure if there is a flag that controls across all.

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