c++/modules: Improve errors for bad module-directives [PR115200]

c++/modules: Improve errors for bad module-directives [PR115200]

This fixes an ICE when a module directive is not given at global scope.
Although not explicitly mentioned, it seems implied from [basic.link] p1
and [module.global.frag] that a module-declaration must appear at the
global scope after preprocessing.  Apart from this the patch also
slightly improves the errors given when accidentally using a module
control-line in other situations where it is not expected.

	PR c++/115200

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_error_1): Special-case unexpected module
	directives for better diagnostics.
	(cp_parser_module_declaration): Check that the module
	declaration is at global scope.
	(cp_parser_import_declaration): Sync error message with that in
	cp_parser_error_1.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/mod-decl-1.C: Update error messages.
	* g++.dg/modules/mod-decl-6.C: New test.
	* g++.dg/modules/mod-decl-7.C: New test.
	* g++.dg/modules/mod-decl-8.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>