openmp: unified_address support

openmp: unified_address support

This makes "requires unified_address" work by making it eqivalent to
"requires unified_shared_memory".  This is more than is strictly necessary,
but should be standard compliant.

gcc/c/ChangeLog:

	* c-parser.cc (c_parser_omp_requires): Check requires unified_address
	for conflict with -foffload-memory=shared.

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_omp_requires): Check requires unified_address
	for conflict with -foffload-memory=shared.

gcc/fortran/ChangeLog:

	* openmp.cc (gfc_match_omp_requires): Check requires unified_address
	for conflict with -foffload-memory=shared.

gcc/ChangeLog:

	* omp-low.cc: Do USM transformations for "unified_address".

gcc/testsuite/ChangeLog:

	* c-c++-common/gomp/usm-4.c: New test.
	* gfortran.dg/gomp/usm-4.f90: New test.