[OpenMP] Implement TR8 `present` motion modifier in Clang (1/2)

Authored by jdenny on Jul 29 2020, 9:18 AM.


[OpenMP] Implement TR8 present motion modifier in Clang (1/2)

This patch implements Clang front end support for the OpenMP TR8
present motion modifier for omp target update directives. The
next patch in this series implements OpenMP runtime support.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D84711