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

[via llvm-commits]

CongzheUalberta wrote:

> Does the debug info always contain the absolute path, or does it contain the relative/absolute path however you invoked `clang`?


I assume you are talking about "remark" info not "debug" info? When clang generates optimization remarks with LTO enabled, the optimization remark dump at the preLTO stage and the LTO stage are not consistent in the sense that remarks at the preLTO stage output absolute file path and remarks at LTO stage output relative file path. So what this patch does is to fix this inconsistent behavior and always output absolute paths for both preLTO and LTO stages. 

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