[OpenMP] Create custom state machines for generic target regions

In the spirit of TRegions [0], this patch creates a custom state
machine for a generic target region based on the potentially called
parallel regions.

The code analysis is done interprocedurally via an abstract attribute
(AAKernelInfo). All outermost parallel regions are collected and we
check if there might be unknown outermost parallel regions for which
we need an indirect call. Other AAKernelInfo extensions are expected.

[0] https://link.springer.com/chapter/10.1007/978-3-030-28596-8_11

Differential Revision: https://reviews.llvm.org/D101977