[OMPIRBuilder] Add support for simd (loop) directive.

This patch adds OMPIRBuilder support for the simd directive (without any clause).  This will be a first step towards lowering simd directive in LLVM_Flang.  The patch uses existing CanonicalLoop infrastructure of IRBuilder to add the support.  Also adds necessary code to add llvm.access.group and llvm.loop metadata wherever needed.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D114379