Suppress -Wwarn-unused-variables when we don't know the constructor

This warning is supposed to be suppressed when the
constructor/destructor are non-trivial, since it might be a RAII type.
However, if the type has a trivial destructor and the constructor hasn't
been resolved (since it is called with dependent arguments), we were
still warning.

This patch suppresses the warning if the type could possibly have a
be a non-trivial constructor call.  Note that this does not take the
arity of the constructors into consideration, so it might suppress
the warning in cases where it isn't possible to call a non-trivial
constructor.