[OpenMP] Add support for close map modifier in Clang

Summary:
This patch adds support for the close map modifier in Clang.

This ensures that the new map type is marked and passed to the OpenMP runtime appropriately.

Additional regression tests have been merged from patch D55892 (author @saghir).

Reviewers: ABataev, caomhin, jdoerfert, kkwli0

Reviewed By: ABataev

Subscribers: kkwli0, Hahnfeld, saghir, guansong, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65341

llvm-svn: 368491