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

[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