[OpenMP] Use the OpenMPIRBuilder for `omp parallel`

This allows to use the OpenMPIRBuilder for parallel regions. Code was
extracted from D61953 and adapted to work with the new version (D70109).

All but one feature should be supported. An update of this patch will
provide test coverage and privatization other than shared.

Reviewed By: fghanim

Differential Revision: https://reviews.llvm.org/D70290