[OPENMP][NVPTX]Added/fixed debugging messages, NFC.

Summary: Added or fixed new/old debugging messages for the better diagnostics.

Reviewers: gtbercea, kkwli0, grokos

Reviewed By: grokos

Subscribers: caomhin, guansong, openmp-commits

Differential Revision: https://reviews.llvm.org/D56102

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@350137 91177308-0d34-0410-b5e6-96231b3b80d8