C++ PATCH for c++/84221, bogus -Wunused with unused attribute on template

[Jason Merrill]

For bugs 54372 and 60063, we changed attributes used and unused to be
applied immediately in a template even if what they apply to is
dependent, to avoid bogus warnings from
maybe_warn_unused_local_typedefs.  But that's only an issue for
TYPE_DECL, so we can use the normal logic for other things.

Tested x86_64-pc-linux-gnu, applying to trunk.
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