librustc: Forbid duplicate name bindings in the same parameter or type

parameter list.

This breaks code like:

    fn f(a: int, a: int) { ... }
    fn g<T,T>(a: T) { ... }

Change this code to not use the same name for a parameter. For example:

    fn f(a: int, b: int) { ... }
    fn g<T,U>(a: T) { ... }

Code like this is *not* affected, since `_` is not an identifier:

    fn f(_: int, _: int) { ... } // OK

Closes #17568.

r? @alexcrichton 
[breaking-change]
This could just be `if !seen_bindings.insert(ident) { ... }`