[llvm] [OpenMP][OMPIRBuilder] Attach `Attribute::OptimizeNone` to user-defined (and default) mappers (PR #174776)

[Kareem Ergawy via llvm-commits]

ergawy wrote:

@jhuber6 thanks a lot for the bisect flag, I did not know about it and helped tremendously! I opened another PR with a hopefully proper fix since I better understand the issue now: https://github.com/llvm/llvm-project/pull/175133.

https://github.com/llvm/llvm-project/pull/174776