OpenMP: Update "declare target"/OpenMP context interaction

OpenMP: Update "declare target"/OpenMP context interaction

The code and test case previously implemented the OpenMP 5.0 spec,
which said in section 2.3.1:

"For functions within a declare target block, the target trait is added
to the beginning of the set..."

In OpenMP 5.1, this was changed to
"For device routines, the target trait is added to the beginning of
the set..."

In OpenMP 5.2 and TR12, it says:
"For procedures that are determined to be target function variants
by a declare target directive..."

The definition of "device routine" in OpenMP 5.1 is confusing, but
certainly the intent of the later versions of the spec is clear that
it doesn't just apply to functions within a begin declare target/end
declare target block.

The only use of the "omp declare target block" function attribute was
to support the 5.0 language, so it can be removed.  This patch changes
the context augmentation to use the "omp declare target" attribute
instead.

gcc/c-family/ChangeLog
	* c-attribs.cc (c_common_gnu_attributes): Delete "omp declare
	target block".

gcc/c/ChangeLog
	* c-decl.cc (c_decl_attributes): Don't add "omp declare target
	block".

gcc/cp/ChangeLog
	* decl2.cc (cplus_decl_attributes): Don't add "omp declare target
	block".

gcc/ChangeLog
	* omp-general.cc (omp_complete_construct_context): Check
	"omp declare target" attribute, not "omp declare target block".

gcc/testsuite/ChangeLog
	* c-c++-common/gomp/declare-target-indirect-2.c : Adjust
	expected output for removal of "omp declare target block".
	* c-c++-common/gomp/declare-variant-8.c: Likewise, the variant
	call to f20 is now resolved differently.
	* c-c++-common/gomp/reverse-offload-1.c: Adjust expected output.
	* gfortran.dg/gomp/declare-variant-8.f90: Likewise, both f18
	and f20 now resolve to the variant.  Delete obsolete comments.