[OpenMP][OpenACC] Implement `ompx_hold` map type modifier extension in runtime (2/2)

This patch implements OpenMP runtime support for an original OpenMP
extension we have developed to support OpenACC: the `ompx_hold` map
type modifier.  The previous patch in this series, D106509, implements
Clang support and documents the new functionality in detail.

Reviewed By: grokos

Differential Revision: https://reviews.llvm.org/D106510