librustc: Take the parameter space into account when combining type

parameters.

This can break code that mistakenly used type parameters in place of
`Self`. For example, this will break:

    trait Foo {
        fn bar<X>(u: X) -> Self {
            u
        }
    }

Change this code to not contain a type error. For example:

    trait Foo {
        fn bar<X>(_: X) -> Self {
            self
        }
    }

Closes #15172.

[breaking-change]

r? @alexcrichton