c++: Fix handling of no-linkage decls for modules

c++: Fix handling of no-linkage decls for modules

When testing the changes for PR c++/112631 we discovered that currently
we don't emit definitions of block-scope function declarations if
they're not used in the module interface TU, which causes issues if they
are used by importers.

This patch fixes the handling of no-linkage declarations for C++20. In
particular, a type declared in a function with vague linkage or declared
in a module CMI could potentially be accessible outside its defining TU,
and as such we can't assume that function declarations using that type
can never be defined in another TU.

A complication with handling this is that we're only strictly interested
in declarations with a module CMI, but when parsing the global module
fragment we don't yet know whether or not this module will have a CMI
until we reach the "export module" line (or not). Since this case is
IFNDR anyway (by [basic.def.odr] p11) we just tentatively assume while
parsing the GMF that this module will have a CMI; once we see (or don't
see) an 'export module' declaration we can commit to that knowledge for
future declarations.

gcc/cp/ChangeLog:

	* cp-tree.h (module_maybe_has_cmi_p): New function.
	* decl.cc (grokfndecl): Mark block-scope functions as public if
	they could be visible in other TUs.
	* decl2.cc (no_linkage_error): Don't error for declarations that
	could be defined in other TUs since C++20. Suppress duplicate
	errors from 'check_global_declaration'.
	* tree.cc (no_linkage_check): In relaxed mode, don't consider
	types in a module CMI to have no linkage.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/linkage-1.C: New test.
	* g++.dg/modules/block-decl-3.h: New test.
	* g++.dg/modules/block-decl-3_a.C: New test.
	* g++.dg/modules/block-decl-3_b.C: New test.
	* g++.dg/modules/block-decl-3_c.C: New test.
	* g++.dg/modules/linkage-1_a.C: New test.
	* g++.dg/modules/linkage-1_b.C: New test.
	* g++.dg/modules/linkage-1_c.C: New test.
	* g++.dg/modules/linkage-2.C: New test.

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