Add enclosing item context when pointing at FnMut bound

[estebank]

Ensure that we always show the fn (and its enclosing trait/impl if appropriate) when pointing at an FnMut bound:

help: `Fn` and `FnMut` closures require captured values to be able to be consumed multiple times, but `FnOnce` closures may consume them only once
  --> $DIR/fnmut-closure-moving-value.rs:11:14
   |
LL | impl Bar {
LL |     fn f<F>(&self, mut f: F)
LL |     where F: FnMut()
   |              ^^^^^^^

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