[OpenMP][NFC] Provide a new remark and documentation

If a GPU function is externally reachable we give up trying to find the
(unique) kernel it is called from. This can hinder optimizations. Emit a
remark and explain mitigation strategies.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D93439

GitOrigin-RevId: 994bb6eb7d01db1d9461e54d17a63af2ba1af2c9
1 file changed