openmp: Add support for OpenMP 5.1 atomics for C++

openmp: Add support for OpenMP 5.1 atomics for C++

Besides the C++ FE changes, I've noticed that the C FE didn't reject
  #pragma omp atomic capture compare
  { v = x; x = y; }
and other forms of atomic swap, this patch fixes that too.  And the
c-family/ routine needed quite a few changes so that the new code
in it works fine with both FEs.

2021-09-17  Jakub Jelinek  <jakub@redhat.com>

gcc/c-family/
	* c-omp.c (c_finish_omp_atomic): Avoid creating
	TARGET_EXPR if test is true, use create_tmp_var_raw instead of
	create_tmp_var and add a zero initializer to TARGET_EXPRs that
	had NULL initializer.  When omitting operands after v = x,
	use type of v rather than type of x.  Fix type of vtmp
	TARGET_EXPR.
gcc/c/
	* c-parser.c (c_parser_omp_atomic): Reject atomic swap if capture
	is true.
gcc/cp/
	* cp-tree.h (finish_omp_atomic): Add r and weak arguments.
	* parser.c (cp_parser_omp_atomic): Update function comment for
	OpenMP 5.1 atomics, parse OpenMP 5.1 atomics and fail, compare and
	weak clauses.
	* semantics.c (finish_omp_atomic): Add r and weak arguments, handle
	them, handle COND_EXPRs.
	* pt.c (tsubst_expr): Adjust for COND_EXPR forms that
	finish_omp_atomic can now produce.
gcc/testsuite/
	* c-c++-common/gomp/atomic-18.c: Expect same diagnostics in C++ as in
	C.
	* c-c++-common/gomp/atomic-25.c: Drop c effective target.
	* c-c++-common/gomp/atomic-26.c: Likewise.
	* c-c++-common/gomp/atomic-27.c: Likewise.
	* c-c++-common/gomp/atomic-28.c: Likewise.
	* c-c++-common/gomp/atomic-29.c: Likewise.
	* c-c++-common/gomp/atomic-30.c: Likewise.  Adjust expected diagnostics
	for C++ when it differs from C.
	(foo): Change return type from double to void.
	* g++.dg/gomp/atomic-5.C: Adjust expected diagnostics wording.
	* g++.dg/gomp/atomic-20.C: New test.
libgomp/
	* testsuite/libgomp.c-c++-common/atomic-19.c: Drop c effective target.
	Use /* */ comments instead of //.
	* testsuite/libgomp.c-c++-common/atomic-20.c: Likewise.
	* testsuite/libgomp.c-c++-common/atomic-21.c: Likewise.
	* testsuite/libgomp.c++/atomic-16.C: New test.
	* testsuite/libgomp.c++/atomic-17.C: New test.

(cherry picked from commit 3a2bcffac602f5de56537a77db1062984bcefd45)