PR c++/54372 - unused attribute inactive on dependant entities

In the example of this patch, gcc/g++ invoked with
-Wunused-local-typedefs warns on dependant entities even when those
are decorated with the 'unused' attribute.

This is because in cplus_decl_attributes, save_template_attributes
makes so that the 'unused' attribute is applied to its appertaining
entity only at instantiation time.  But then at parsing time
maybe_warn_unused_local_typedefs checks for TREE_USED before warning.

This patch applies the 'unused' attribute at compilation time.

Tested on x86_64-unknown-linux-gnu against trunk.

gcc/cp/

	* decl2.c (is_late_template_attribute): "unused" attribute is to
	be applied at compile time.

gcc/testsuite/

	* c-c++-common/Wunused-local-typedefs-2.c: New test.

From-SVN: r191830