[OpenMP] Implement TR8 `present` map type modifier in runtime (2/2)

This implements OpenMP runtime support for the OpenMP TR8 `present`
map type modifier.  The previous patch in this series implements Clang
front end support.  See that patch summary for behaviors that are not
yet supported.

Reviewed By: grokos, jdoerfert

Differential Revision: https://reviews.llvm.org/D83062