openmp: Handle OpenMP directives in attribute syntax in attribute-declaration

openmp: Handle OpenMP directives in attribute syntax in attribute-declaration

Now that we parse attribute-declaration (outside of functions), the following
patch handles OpenMP directives in its attribute(s).
What needs handling incrementally is diagnose mismatching begin/end pair
like
 [[omp::directive (declare target)]];
 int a;
 #pragma omp end declare target
or
 #pragma omp declare target
 int b;
 [[omp::directive (end declare target)]];
and handling declare simd/declare variant on declarations (function
definitions and declarations), for those in two different spots.

2021-07-31  Jakub Jelinek  <jakub@redhat.com>

	* parser.c (cp_parser_declaration): Handle OpenMP directives
	in attribute-declaration.

	* g++.dg/gomp/attrs-9.C: New test.