Provide more context when an expected type was a type parameter

[estebank]

This becomes particularly confusing when a type parameter has a default.

  --> $DIR/default-type-parameter.rs:8:17
   |
LL |     fn foo() -> u32 { 0 }
   |                 ^^^ expected `()`, found `u32`
   |
note: type in trait
  --> $DIR/default-type-parameter.rs:2:17
   |
LL |     fn foo() -> T;
   |                 ^
   = note: expected signature `fn() -> ()`
              found signature `fn() -> u32`
note: expected to match this type parameter
  --> $DIR/default-type-parameter.rs:1:13
   |
LL | trait Trait<T = ()> {
   |             ^^^^^^
help: change the output type to match the trait
   |
LL -     fn foo() -> u32 { 0 }
LL +     fn foo() -> () { 0 }

Fix #67991.