Issue Description:

In the following example, we suggest both `b` and `c` as potential argument labels. However, only `b` should be suggested because the first parameter is an `Int` and thus the second definition of `foo` is not applicable.

func foo(a: Int, b: Int) {}
func foo(a: String, c: Int) {}
foo(a: 1, #^COMPLETE^#)

Fixing this is non-trivial because it means that we’d need to type-check the incomplete expression when completing the argument labels, which we don’t do today.