libgomp, Fortran: Fix OpenACC "gang reduction on an orphan loop" error message

libgomp, Fortran: Fix OpenACC "gang reduction on an orphan loop" error message

According to the OpenACC standard version 2.5 and later, reductions on
orphaned gang loops are explicitly disallowed (cf.  section "Changes
from Version 2.0 to 2.5").  A loop is "orphaned" if it is not
lexically contained in a compute construct (cf. section "Loop
construct" of the OpenACC standard), i.e. in either a "parallel", a
"serial", or a "kernels" construct.

This commit fixes the check for reductions on orphaned gang loops in
the Fortran frontend which (in contrast to the C, C++ frontends)
erroneously rejects reductions on gang loops that are contained in
"kernels" constructs.

2020-07-20  Frederik Harwath  <frederik@codesourcery.com>

gcc/fortran/

	* openmp.c (oacc_is_parallel_or_serial): Removed function.
	(oacc_is_kernels): New function.
	(oacc_is_compute_construct): New function.
	(resolve_oacc_loop_blocks): Use "oacc_is_compute_construct"
	instead of "oacc_is_parallel_or_serial" for checking that a
	loop is not orphaned.

gcc/testsuite/

	* gfortran.dg/goacc/orphan-reductions-2.f90: New test
	verifying that the "gang reduction on an orphan loop" error message
	is not emitted for non-orphaned loops.

	* c-c++-common/goacc/orphan-reductions-2.c: Likewise for C and C++.