openmp: allow requires unified_shared_memory

openmp: allow requires unified_shared_memory

This is the front-end portion of the Unified Shared Memory implementation.
It checks that -foffload-memory isn't set to an incompatible mode.

Backport of the patch posted at
https://gcc.gnu.org/pipermail/gcc-patches/2022-March/591351.html

gcc/c/ChangeLog:

	* c-parser.c (c_parser_omp_requires): Check compatibility of
	-foffload-memory option with requires directive.

gcc/cp/ChangeLog:

	* parser.c (cp_parser_omp_requires): Check compatibility of
	-foffload-memory option with requires directive.

gcc/fortran/ChangeLog:

	* openmp.c (gfc_match_omp_requires): Check compatibility of
	-foffload-memory option with requires directive.

gcc/testsuite/ChangeLog:

	* c-c++-common/gomp/usm-1.c: New test.
	* gfortran.dg/gomp/usm-1.f90: New test.