Suggest similarly named assoc items in trait impls

Previously, the compiler didn't suggest similarly named associated items
unlike we do in many situations. This patch adds such diagnostics for
associated functions, types and constants.

src/test/ui/error-codes/E0407.stderr

@@ -2,7 +2,10 @@ error[E0407]: method `b` is not a member of trait `Foo`
   --> $DIR/E0407.rs:9:5
    |
 LL |     fn b() {}
-   |     ^^^^^^^^^ not a member of trait `Foo`
+   |     ^^^-^^^^^
+   |     |  |
+   |     |  help: there is an associated function with a similar name: `a`
+   |     not a member of trait `Foo`
 
 error: aborting due to previous error