OpenMP (C only): omp allocate - extend parsing support, improve diagnostic

OpenMP (C only): omp allocate - extend parsing support, improve diagnostic

The 'allocate' directive can be used for both stack and static variables.
While the parser in C and C++ was pre-existing, it missed several
diagnostics, which this commit adds - for now only for C.

While the "sorry, unimplemented" for static variables is still issues
during parsing, the sorry for stack variables is now issued in the
middle end, preparing for the actual implementation. (Again: only for C.)

gcc/c/ChangeLog:

	* c-parser.cc (c_parser_omp_construct): Move call to
	c_parser_omp_allocate to ...
	(c_parser_pragma): ... here.
	(c_parser_omp_allocate): Avoid ICE is allocator could not be
	parsed; set 'omp allocate' attribute for stack/automatic variables
	and only reject static variables; add several additional
	restriction checks.
	* c-tree.h (c_mark_decl_jump_unsafe_in_current_scope): New prototype.
	* c-decl.cc (decl_jump_unsafe): Return true for omp-allocated decls.
	(c_mark_decl_jump_unsafe_in_current_scope): New.
	(warn_about_goto, c_check_switch_jump_warnings): Add error for
	omp-allocated decls.

gcc/ChangeLog:

	* gimplify.cc (gimplify_bind_expr): Check for
	insertion after variable cleanup.  Convert 'omp allocate'
	var-decl attribute to GOMP_alloc/GOMP_free calls.

gcc/testsuite/ChangeLog:

	* c-c++-common/gomp/allocate-5.c: Fix testcase; make some
	dg-messages for 'sorry' as c++, only.
	* c-c++-common/gomp/directive-1.c: Make a 'sorry' c++ only.
	* c-c++-common/gomp/allocate-9.c: New test.
	* c-c++-common/gomp/allocate-11.c: New test.
	* c-c++-common/gomp/allocate-12.c: New test.
	* c-c++-common/gomp/allocate-14.c: New test.
	* c-c++-common/gomp/allocate-15.c: New test.
	* c-c++-common/gomp/allocate-16.c: New test.