c++: Accept C++11 attribute-definition [PR101582]

c++: Accept C++11 attribute-definition [PR101582]

As the following testcase shows, we don't parse properly
C++11 attribute-declaration:
https://eel.is/c++draft/dcl.dcl#nt:attribute-declaration

cp_parser_toplevel_declaration just handles empty-declaration parsing
(with diagnostics for C++98) and otherwise calls cp_parser_declaration
which on it calls cp_parser_simple_declaration and rejects it with
"does not declare anything" permerror.

The following patch moves the handling of empty-declaration from
cp_parser_toplevel_declaration to cp_parser_declaration and
handles attribute-declaration in cp_parser_declaration
by parsing the attributes (standard ones only, we've never supported
__attribute__((...)); at namespace scope, so I'm not sure we need to
introduce that), which for C++98 emits the needed diagnostics, and then
warning if there are any attributes that we throw away on the floor.

I'll need this later for OpenMP directives at namespace scope, e.g.
[[omp::directive (requires, atomic_default_mem_order(seq_cst))]];
should be valid at namespace scope (and many other directives).

2021-07-30  Jakub Jelinek  <jakub@redhat.com>

	PR c++/101582
	* parser.c (cp_parser_skip_std_attribute_spec_seq): Add a forward
	declaration.
	(cp_parser_declaration): Parse empty-declaration and
	attribute-declaration.
	(cp_parser_toplevel_declaration): Don't parse empty-declaration here.

	* g++.dg/cpp0x/gen-attrs-45.C: Expect a warning about ignored
	attributes instead of error.
	* g++.dg/cpp0x/gen-attrs-75.C: New test.
	* g++.dg/modules/pr101582-1.C: New test.