[OpenMP] Use the OpenMPIRBuilder for omp cancel
An omp cancel parallel needs to be emitted by the OpenMPIRBuilder if
the parallel was emitted by the OpenMPIRBuilder. This patch makes
this possible. The cancel logic is shared with the cancel barriers.
Testing is done via unit tests and the clang cancel_codegen.cpp file
once D70290 lands.
Reviewed By: JonChesterfield
Differential Revision: https://reviews.llvm.org/D71948