[OpenMP][OMPIRBuilder] Introducing the `OMPBuilderCBHelpers` helper class

This patch introduces a new helper class `OMPBuilderCBHelpers`,
which will contain all reusable C/C++ language specific function-
alities required by the `OMPIRBuilder`.

Initially, this helper class contains the body and finalization
codegen functionalities implemented using callbacks which were
moved here for reusability among the different directives
implemented in the `OMPIRBuilder`, along with RAIIs for preserving
state prior to emitting outlined and/or inlined OpenMP regions.

In the future this helper class will also contain all the different
call backs required by OpenMP clauses/variable privatization.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D74562