[Typechecker] Covariant subscript check must also take into consideration mismatched types

[theblixguy]

When we override a settable subscript with a different argument type, the compiler emits an error complaining that the types are not covariant:

class A {
  subscript (foo: Int) -> Int {
    get { fatalError() }
    set {}
  }
}
class B: A {
  override subscript (foo: String) -> Int {
    get { fatalError() }
    set {}
  }
}

The compiler currently emits an error saying cannot override mutable subscript of type '(Int) -> Int' with covariant type '(String) -> Int', but the argument types (Int and String) are unrelated.

We should make sure that we don't have mismatched types, so we can print the right diagnostics.

Resolves SR-10323

[theblixguy]

@swift-ci please smoke test