debug/123376 - mangle decls referenced in initializers early

The following makes sure to mangle decls referenced in initializers,
even when of aggregate type, during the early debug phase since
later we eventually leave stray supposedly unused CV qualified
types as their own main variant which confuses C++ mangling.  The
comment that refers to rtl_for_decl_init punting might be
accurate, but loc_list_from_tree_1 will happily see to
cst_pool_loc_descr where constant pool lookup will eventually
create DECL_RTL of referenced symbols, triggering mangling.

	PR debug/123376
	* dwarf2out.cc (tree_add_const_value_attribute): Walk all
	initializers for early mangling of referenced decls.
	(mangle_referenced_decls): Also walk subtrees of CONSTRUCTORS.

	* g++.dg/lto/pr123376_0.C: New testcase.