Point at local similarly named element and tweak references to variants

Point at the span for the definition of ADTs internal to the current
crate.

Look at the leading char of the ident to determine whether we're
expecting a likely fn or any of a fn, a tuple struct or a tuple variant.

Turn fn `add_typo_suggestion` into a `Resolver` method.

src/test/ui/methods/method-resolvable-path-in-pattern.rs

@@ -9,6 +9,6 @@ impl MyTrait for Foo {}
 fn main() {
     match 0u32 {
         <Foo as MyTrait>::trait_bar => {}
-        //~^ ERROR expected unit struct/variant or constant, found method `MyTrait::trait_bar`
+        //~^ ERROR expected unit struct, unit variant or constant, found method `MyTrait::trait_bar`
     }
 }