Remove unused variables '__kmp_build_check_*' for non assert builds.

Add new LIBOMP_ENABLE_ASSERTIONS macro which can be set in a standalone build
or takes the value of LLVM_ENABLE_ASSERTIONS when inside llvm/projects.  This
change also defines the KMP_BUILD_ASSERT() macro to do nothing when ENABLE_ASSERTIONS
is off.  This means the __kmp_build_check_* types won't be defined and thus, no warnings.
http://lists.cs.uiuc.edu/pipermail/openmp-dev/2015-June/000719.html

Patch by Jack Howarth and Jonathan Peyton

llvm-svn: 239546