[OpenMP] Cleanup code

This patch cleans up unused functions, variables, sign compare issues, and
addresses some -Warning flags which are now enabled including -Wcast-qual.
Not all the warning flags in LibompHandleFlags.cmake are enabled, but some
are with this patch.

Some __kmp_gtid_from_* macros in kmp.h are switched to static inline functions
which allows us to remove the awkward definition of KMP_DEBUG_ASSERT() and
KMP_ASSERT() macros which used the comma operator. This had to be done for the
innumerable -Wunused-value warnings related to KMP_DEBUG_ASSERT()

Differential Revision: https://reviews.llvm.org/D49105


git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@339393 91177308-0d34-0410-b5e6-96231b3b80d8