openmp: Actually ignore pragma_stmt pragmas for which c_parser_pragma returns false

Unlike the C++ FE, the C FE ignored pragmas (as if they weren't there) in
pragma_stmt contexts if c*_parser_pragma returns false only when after labels,
not inside of substatements of selection or loop statements.
After making just that change, several gomp/goacc testcases started failing,
because extra diagnostics has been emitted (in C, in C++ it was emitted
already before).  Say
void
foo (int x)
{
  if (x)
    #pragma omp barrier
}
used to in C emit just an error that the pragma is not allowed in such
contexts, but in C++ emitted both that and a parsing error that
  if (x)
}
is invalid.  So, the rest of this patch is mostly about returning true
after we report that that certain pragma is not allowed in pragma_stmt
contexts, because for error-recovery it seems better to treat the
pragma in that case as something that is the substatement of such if etc.
c*_parser_pragma return value is only ever used for pragma_stmt context,
in which false means act as if the pragma isn't there (e.g. has been handled
already by preprocessor etc.), and true which means it was there.

2021-08-18  Jakub Jelinek  <jakub@redhat.com>

gcc/c/
	* c-parser.c (c_parser_statement_after_labels): Add restart label
	near the start of the function.  If c_parser_pragma returns false,
	goto restart.
	(c_parser_pragma): For PRAGMA_OMP_CANCELLATION_POINT return what
	c_parser_omp_cancellation_point returned.  For PRAGMA_OMP_DECLARE
	return what c_parser_omp_declare returned.  Return true instead of
	false after emitting errors that the directive is not allowed in
	pragma_stmt context.
	(c_parser_omp_ordered): Return true instead of
	false after emitting errors that the directive is not allowed in
	pragma_stmt context.
	(c_parser_omp_target_update): Likewise.
	(c_parser_omp_target_enter_data, c_parser_omp_target_exit_data):
	Change return type from tree to bool, return false if the
	directive should be ignored in pragma_stmt contexts.
	(c_parser_omp_target): Adjust callers of c_parser_omp_target_*_data,
	return their result directly.
	(c_parser_omp_cancellation_point): Change return type from void to
	bool, return false if the directive should be ignored in pragma_stmt
	contexts.
	(c_parser_omp_declare): Likewise.
gcc/cp/
	* parser.c (cp_parser_omp_ordered): Return true instead of
	false after emitting errors that the directive is not allowed in
	pragma_stmt context.
	(cp_parser_omp_target_update): Likewise.
	(cp_parser_omp_cancellation_point): Change return type from void to
	bool, return false if the directive should be ignored in pragma_stmt
	contexts.
	(cp_parser_omp_target_enter_data, cp_parser_omp_target_exit_data):
	Change return type from tree to bool, return false if the
	directive should be ignored in pragma_stmt contexts.
	(cp_parser_omp_target): Adjust callers of cp_parser_omp_target_*_data,
	return their result directly.
	(cp_parser_pragma): For PRAGMA_OMP_CANCELLATION_POINT return what
	cp_parser_omp_cancellation_point returned.  Return true instead of
	false after emitting errors that the directive is not allowed in
	pragma_stmt context.
gcc/testsuite/
	* c-c++-common/gomp/pr63326.c: Don't expect extra "before" errors
	in C++.
	* g++.dg/gomp/attrs-7.C: Don't expect one extra error.
	* g++.dg/gomp/barrier-2.C: Likewise.
	* gcc.dg/gomp/declare-simd-5.c: Likewise.
	* gcc.dg/gomp/barrier-2.c: Likewise.
	* gcc.dg/gomp/declare-variant-2.c: Likewise.