[OpenMP] Misc. changes that add or remove pointer/bound checks
- Added or moved checks to appropriate places.
- Removed ineffective null check where the pointer is already being dereferenced around the code.
- Initialized variables that can be used without definitions.
- Added call to dlclose/FreeLibrary in OMPT tool activation.
- Added a new build compiler definition.
Differential Revision: https://reviews.llvm.org/D98584