This patch implements the code generation to use OpenMP 5.0 declare mapper (a.k.a. user-defined mapper) constructs.
Patch written by Lingda Li.
Differential Revision: https://reviews.llvm.org/D67833
If the ancestor device modifier is used and the value of the device
clause is evaluated to 1, the ancestor device shall be used for the
execution.
Since the reverse offloading is not supported yet, the target construct
execution is always initiated from the host, not from the device. So, if
the ancestor modifier is specified, just execute target region on the
host.