[OpenMP][OMPIRBuilder] Add Directives (master and critical) to OMPBuilder.
Add support for Master and Critical directive in the OMPIRBuilder. Both make use of a new common interface for emitting inlined OMP regions called emitInlinedRegion which was added in this patch as well.
Also this patch modifies clang to use the new directives when -fopenmp-enable-irbuilder commandline option is passed.
Reviewed By: jdoerfert
Differential Revision: https://reviews.llvm.org/D72304