in which the fn-must-use codepath is prevented from panicking on closure

The must-use lint needs the DefId of called functions and method
receivers in order to look for a `#[must_use]` attribute, but this would
ICE (!) if a called function was actually a closure (with a non-unit
return value). Instead, let's be specific that we want a `Def::Fn`,
rather than blithely assuming that we can get the DefId of a qpath.

Supporting must-use closures doesn't seem like a priority, but could
conceivably be added in the future if desired (conditional on the
statement and expression attributes (#15701) story being amicable).

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