Hi Thomas,
this patch implements the OpenACC 2.7 change requiring the host_data construct
to have at least one use_device clause.

This patch started out with a simple check during gimplify (much smaller patch),
but turned out that front-ends removed use_device clauses when they have error,
and the gimplify check started to echo a "no use_device clause" message in such
cases, which seem confusing for the user. So ended up adding the check in each
front-end instead.

Tested on powerpc64le-linux/nvptx, x86_64-linux/amdgcn tests in progress (expect
no surprises). Is this okay for trunk?

Thanks,
Chung-Lin

gcc/c/ChangeLog:

	* c-parser.cc (c_parser_oacc_host_data): Add checking requiring OpenACC
	host_data construct to have an use_device clause.

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_oacc_host_data): Add checking requiring OpenACC
	host_data construct to have an use_device clause.

gcc/fortran/ChangeLog:

	* trans-openmp.cc (gfc_trans_oacc_construct): Add checking requiring
	OpenACC host_data construct to have an use_device clause.

gcc/testsuite/ChangeLog:

	* c-c++-common/goacc/host_data-2.c: Adjust testcase.
	* gfortran.dg/goacc/host_data-error.f90: New testcase.
	* gfortran.dg/goacc/pr71704.f90: Adjust testcase.