Sema: Check worseThanBestSolution() in a better place

[slavapestov]

This change speeds up this test case, but it remains exponential:

let x: Int? = nil

func f<T>(_: T?, _: T) -> T {}
func f<T>(_: T?, _: T?) -> T? {}

let _ = [f(x, 0), f(x, 0), f(x, 0), f(x, 0), f(x, 0), f(x, 0), f(x, 0), f(x, 0)]

Also, clean up DeepEquality for protocol compositions. This is one of the blockers towards enabling protocol compositions that contain parameterized protocol types, like any Sequence<Int> & Sendable, etc.

[slavapestov]

@swift-ci Please test source compatibility

[slavapestov]

@swift-ci Please smoke test