LLVM_PROFILE_FILE env override flag

Hey I'm working on a pretty big C++ project and we're doing some performance testing for due diligence and I was wondering if there's a flag that you can set that hardcodes the `.profraw` file name for a given executable

I thought `-fprofile-instrument-path` would have worked but it didn't

I've got a really weird constraint where the program is launched by a daemon process and somewhere somehow the `LLVM_PROFILE_FILE` env var gets overwritten and the application it launches ends up not generating any `.profraw` stuff whatsoever

If the feature does not exist, I would be happy to implement it :)

However if it does, and I failed to properly apply RTFM to my work, that would be pretty great too!

Thanks in Advance

Start with LLVM's profiling documentation and the existing handling of LLVM_PROFILE_FILE and -fprofile-instrument-path. Determine whether an executable-specific .profraw filename override already exists; if not, define the requested flag's behavior and verify that it still works when the daemon overwrites the environment variable.

Written by the indexing model from the issue text.