`heterogeneous collection literal could only be inferred` warning for a homogeneous collection literal

[simanerush]

Consider the following code:

class Base { }
class Derived : Base { }

let arr: [Base]

for case _ as [Derived] in [arr] {} // warning: heterogeneous collection literal could only be inferred to '[[Base]]'; add explicit type annotation if this is intentional

This diagnostics should not be emitted.