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

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

> After this patch the last line will output the full path as well, i.e., "/home/congzhe/develop/app1/funcs3.cpp:5:2....", so that the behavior becomes consistent.

Can you share what the current behavior is without LTO? (Sorry, my dev machine was recently knocked offline, otherwise I'd check myself).

If w/o LTO we have relative paths, then w/ LTO we should only emit relative paths and this patch should be changed.

If w/o LTO we have absolute paths, then this patch LGTM.

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