Support extensions in functional supertypes

Under a flag for now.

compiler/testData/codegen/box/extensionFunctions/extensionFunctionAsSupertype.kt

@@ -0,0 +1,15 @@
+// !LANGUAGE: +FunctionalTypeWithExtensionAsSupertype
+// IGNORE_BACKEND: JS, JS_IR, WASM
+
+interface I: (String) -> String
+
+class C: String.() -> String, I {
+    override fun invoke(p1: String): String = p1
+}
+
+fun box(): String {
+    val c = C()
+    if (c("OK") != "OK") return c("OK")
+    val ext: String.() -> String = c
+    return "OK".ext()
+}

compiler/testData/diagnostics/tests/subtyping/extFunctionTypeAsSuperTypeRestrictionLifted.kt

@@ -0,0 +1,27 @@
+// FIR_IDENTICAL
+// !LANGUAGE: +FunctionalTypeWithExtensionAsSupertype
+// SKIP_TXT
+class A : Double.() -> Unit {
+    override fun invoke(p1: Double) {}
+}
+
+class B : Int.(Double) -> Unit {
+    override fun invoke(p1: Int, p2: Double) {}
+}
+
+open class C {}
+
+abstract class A0 : C.() -> Int
+abstract class A1 : C.(Int) -> Int
+abstract class A2 : C.(Int, String) -> Int
+
+open class D<T> {}
+
+abstract class B0<T> : D<T>.() -> Int
+abstract class B1<T> : D<T>.(C) -> Int
+abstract class B2<T> : D<T>.(T, C) -> T
+
+interface E<T> {}
+
+abstract class C0: C(), Int.() -> Double
+abstract class C1<T>: C(), E<T>, Int.(C) -> Double