Specify what's going on when reporting "X not assignable to X" error for identical X

[RyanCavanaugh]

From #12005 (comment)

We can sometimes get into a state where we issue errors that appear to be nonsense:

interface T { }
declare const a: T;
class Foo<T> {
    x: T;
    fn() {
        this.x = a;
    }
}

When two types have the same spellings according to typeToString, we should issue a more specific error, e.g.

"Type T is not assignable to type T. Two different types with this name exist, but they are unrelated."