Use span context for enclosing item(s) of unmet bound

[estebank]

Point at item that introduced an unmet bound and in the case of associated items, their container (trait/impl), without using a span label. This adds bits of code to the diagnostic, without explicitly pointing to it.

error[E0277]: cannot add `<T as SubEncoder>::ActualSize` to `<T as SubEncoder>::ActualSize`
  --> $DIR/issue-54108.rs:23:17
   |
LL |     type Size = <Self as SubEncoder>::ActualSize;
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no implementation for `<T as SubEncoder>::ActualSize + <T as SubEncoder>::ActualSize`
   |
   = help: the trait `Add` is not implemented for `<T as SubEncoder>::ActualSize`
note: required by a bound in `Encoder::Size`
  --> $DIR/issue-54108.rs:8:16
   |
LL | pub trait Encoder {
LL |     type Size: Add<Output = Self::Size>;
   |                ^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Encoder::Size`

r? @oli-obk