openmp: Adjust handling of __has_attribute (omp::directive)/sequence and add omp...

openmp: Adjust handling of __has_attribute (omp::directive)/sequence and add omp::decl

I forgot to tweak c_common_has_attribute for the C++ omp::decl addition and now
also for the C omp::{directive,sequence,decl} addition.

2023-11-05  Jakub Jelinek  <jakub@redhat.com>

	* c-lex.cc (c_common_has_attribute): Return 1 for omp::directive
	and omp::sequence with -fopenmp or -fopenmp-simd also for C, not
	just for C++.  Return 1 for omp::decl with -fopenmp or -fopenmp-simd
	for both C and C++.

	* c-c++-common/gomp/attrs-1.c: Adjust for omp::directive and
	omp::sequence being supported also in C and add tests for omp::decl.
	* c-c++-common/gomp/attrs-2.c: Likewise.
	* c-c++-common/gomp/attrs-3.c: Add tests for omp::decl.