Account for tail expressions when pointing at return type

When there's a type mismatch we make an effort to check if it was
caused by a function's return type. This logic now makes sure to
only point at the return type if the error happens in a tail
expression.

src/test/ui/struct-literal-variant-in-if.stderr

@@ -49,9 +49,6 @@ LL |     if x == E::V { field } {}
 error[E0308]: mismatched types
   --> $DIR/struct-literal-variant-in-if.rs:10:20
    |
-LL | fn test_E(x: E) {
-   |                 - help: try adding a return type: `-> bool`
-LL |     let field = true;
 LL |     if x == E::V { field } {}
    |                    ^^^^^ expected (), found bool
    |