lto/114501 - missed free-lang-data for CONSTRUCTOR index

The following makes sure to also walk CONSTRUCTOR element indexes
which can be FIELD_DECLs, referencing otherwise unused types we
need to clean.  walk_tree only walks CONSTRUCTOR element data.

	PR lto/114501
	* ipa-free-lang-data.cc (find_decls_types_r): Explicitly
	handle CONSTRUCTORs as walk_tree handling of those is
	incomplete.

	* g++.dg/pr114501_0.C: New testcase.