[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