[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