Add tests for keypaths converted to funcs with inout param

Author: Jumhyn

test/expr/unary/keypath/keypath.swift

@@ -1258,3 +1258,17 @@ func f(_ arr: [Int?]) {
   let _: [Int] = arr.compactMap(\.?)
   let _: [Int] = arr.map(\.!)
 }
+
+func testKeyPathInout() {
+  let _: (inout String) -> Int = \.count
+  let _: (inout String) -> Int = \String.count
+  let _: (inout String) -> Int? = \.count
+
+  func takesInout(_: (inout String) -> Int) {}
+  func takesInoutOpt(_: (inout String) -> Int?) {}
+
+  takesInout(\.count)
+  takesInout(\String.count)
+  takesInoutOpt(\.count)
+  takesInoutOpt(\String.count)
+}