c++: Follow module grammar more closely [PR110808]

This patch cleans up the parsing of module-declarations and
import-declarations to more closely follow the grammar defined by the
standard.

For instance, currently we allow declarations like 'import A:B', even
from an unrelated source file (not part of module A), which causes
errors in merging declarations. However, the syntax in [module.import]
doesn't even allow this form of import, so this patch prevents this from
parsing at all and avoids the error that way.

Additionally, we sometimes allow statements like 'import :X' or
'module :X' even when not in a named module, and this causes segfaults,
so we disallow this too.

	PR c++/110808

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_module_name): Rewrite to handle
	module-names and module-partitions independently.
	(cp_parser_module_partition): New function.
	(cp_parser_module_declaration): Parse module partitions
	explicitly. Don't change state if parsing module decl failed.
	(cp_parser_import_declaration): Handle different kinds of
	import-declarations locally.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/part-hdr-1_c.C: Fix syntax.
	* g++.dg/modules/part-mac-1_c.C: Likewise.
	* g++.dg/modules/mod-invalid-1.C: New test.
	* g++.dg/modules/part-8_a.C: New test.
	* g++.dg/modules/part-8_b.C: New test.
	* g++.dg/modules/part-8_c.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>