[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