[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

llvm-svn: 350137