C++ PATCH for c++/64251 (unnecessary template instantiation)

[Jason Merrill]

Here we were seeing errors because of template instantiation triggered 
by mark_used within fold_non_dependent_expr.  We don't want to 
instantiate templates based on uses in other uninstantiated templates, 
and we were already trying to avoid that; let's add another check to 
avoid it.

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