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Issue Description:

When attempting to use the identity KeyPath literal (.self) as a function according to functionality in SE-0249, the code fails to compile. Example:

let x: [Int?] = [1, nil, 2]
let x2 = x.compactMap(\.self) // doesn't work -> "Key path value type 'Optional<_>' cannot be converted to contextual type '_'"
let x3: [Int] = x.compactMap(\Int?.self) // doesn't work -> "Cannot convert value of type 'WritableKeyPath<Int?, Int?>' to expected argument type '(Int?) throws -> Int?'"

However if one assigns an identity KeyPath to a variable of type (Root) -> Root, it succeeds:

let xFunc: (Int?) -> Int? = \Int?.self
let x4 = x.compactMap(xFunc) // works

It also succeeds if we define an operator on KeyPath:

prefix operator ^
prefix func ^ <Root, Value>(_ kp: KeyPath<Root, Value>) -> (Root) -> Value { { root in root[keyPath: kp] } }

let x5 = x.compactMap(^\.self) // works

More examples can be found on the attached playground.

From my very basic understanding of this (and please excuse me if I'm saying something stupid!), could it be that the Identity KeyPath's special handling (from SE-0227 is not playing well with the new logic implemented in SE-0249 to convert KeyPath's to functions? Because if .self is translated to a MutableKeyPath<T, T> and MutableKeyPath: KeyPath, it should work like any other KeyPath.

Thanks!