[OpenMP][OMPIRBuilder] Add a configuration class that captures flags that affect codegen

This patch introudces the OpenMPIRBuilderConfig class which contains various
flags that are needed to lower OMP constructs to LLVM-IR. The purpose is to
keep the flags in one place so they do not have to be passed in every time.
The flags can be set optionally since some uses cases don't rely on functions
that depend on these flags.

Reviewed By: jdoerfert, tschuett

Differential Revision: https://reviews.llvm.org/D138220