c++: Redetermine whether to write vtables on stream-in [PR114229]

c++: Redetermine whether to write vtables on stream-in [PR114229]

We currently always stream DECL_INTERFACE_KNOWN, which is needed since
many kinds of declarations already have their interface determined at
parse time.  But for vtables and type-info declarations we need to
re-evaluate on stream-in as whether they need to be emitted or not
changes in each TU, so this patch clears DECL_INTERFACE_KNOWN on these
kinds of declarations so that they can go through 'import_export_decl'
again.

Note that the precise details of the virt-2 tests will need to change
when we implement the resolution of [1], for now I just updated the test
to not fail with the new (current) semantics.

[1]: https://github.com/itanium-cxx-abi/cxx-abi/pull/171

	PR c++/114229

gcc/cp/ChangeLog:

	* module.cc (trees_out::core_bools): Redetermine
	DECL_INTERFACE_KNOWN on stream-in for vtables and tinfo.
	* decl2.cc (import_export_decl): Add fixme for ABI changes with
	module vtables and tinfo.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/virt-2_b.C: Update test to acknowledge that we
	now emit vtables here too.
	* g++.dg/modules/virt-3_a.C: New test.
	* g++.dg/modules/virt-3_b.C: New test.
	* g++.dg/modules/virt-3_c.C: New test.
	* g++.dg/modules/virt-3_d.C: New test.

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