[OpenMP] Code cleanup for NVPTX OpenMP codegen

This patch cleans up private methods for NVPTX OpenMP codegen. It converts private
members to static functions to follow the coding style of CGOpenMPRuntime.cpp and
declutter the header file.

Reviewers: ABataev
Differential Revision: https://reviews.llvm.org/D28124

llvm-svn: 290904