openacc: Set bias to zero for explicit attach/detach clauses in C and C++

openacc: Set bias to zero for explicit attach/detach clauses in C and C++

This is a fix for the pointer (or array) size inadvertently being used
for the bias with attach and detach mapping kinds, for both C and C++.

2020-07-09  Julian Brown  <julian@codesourcery.com>
	    Thomas Schwinge  <thomas@codesourcery.com>

gcc/c/
	PR middle-end/95270
	* c-typeck.c (c_finish_omp_clauses): Set OMP_CLAUSE_SIZE (bias) to zero
	for standalone attach/detach clauses.

gcc/cp/
	PR middle-end/95270
	* semantics.c (finish_omp_clauses): Likewise.

include/
	PR middle-end/95270
	* gomp-constants.h (gomp_map_kind): Expand comment for attach/detach
	mapping kinds.

gcc/testsuite/
	PR middle-end/95270
	* c-c++-common/goacc/mdc-1.c: Update expected dump output for zero
	bias.

libgomp/
	PR middle-end/95270
	* testsuite/libgomp.oacc-c-c++-common/pr95270-1.c: New test.
	* testsuite/libgomp.oacc-c-c++-common/pr95270-2.c: New test.