c++: Propagate TREE_ADDRESSABLE in fixup_type_variants [PR115062]

This has caused issues with modules when an import fills in the
definition of a type already created with a typedef.

	PR c++/115062

gcc/cp/ChangeLog:

	* class.cc (fixup_type_variants): Propagate TREE_ADDRESSABLE.
	(finish_struct_bits): Cleanup now that TREE_ADDRESSABLE is
	propagated by fixup_type_variants.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/pr115062_a.H: New test.
	* g++.dg/modules/pr115062_b.H: New test.
	* g++.dg/modules/pr115062_c.C: New test.

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