OpenMP: Add ME support for 'omp allocate' stack variables

OpenMP: Add ME support for 'omp allocate' stack variables

Call GOMP_alloc/free for 'omp allocate' allocated variables. This is
for C only as C++ and Fortran show a sorry already in the FE. Note that
this only applies to stack variables as the C FE shows a sorry for
static variables.

gcc/ChangeLog:

	* gimplify.cc (gimplify_bind_expr): Call GOMP_alloc/free for
	'omp allocate' variables; move stack cleanup after other
	cleanup.
	(omp_notice_variable): Process original decl when decl
	of the value-expression for a 'omp allocate' variable is passed.
	* omp-low.cc (scan_omp_1_op): Handle 'omp allocate' variables

libgomp/ChangeLog:

	* libgomp.texi (OpenMP 5.1 Impl.): Mark 'omp allocate' as
	implemented for C only.
	* testsuite/libgomp.c/allocate-4.c: New test.
	* testsuite/libgomp.c/allocate-5.c: New test.
	* testsuite/libgomp.c/allocate-6.c: New test.

gcc/testsuite/ChangeLog:

	* c-c++-common/gomp/allocate-11.c: Remove C-only dg-message
	for 'sorry, unimplemented'.
	* c-c++-common/gomp/allocate-12.c: Likewise.
	* c-c++-common/gomp/allocate-15.c: Likewise.
	* c-c++-common/gomp/allocate-9.c: Likewise.
	* c-c++-common/gomp/allocate-10.c: New test.
	* c-c++-common/gomp/allocate-17.c: New test.