K2: Do not use KFunctionN as representation type for adapted references

Beside some corner cases, it's already prohibited in K1 because
adaptation have a bit strange nature
(they don't represent any existing real function exactly)

^KT-55137 Fixed

compiler/testData/codegen/box/fir/callableReferenceConversionTopLevel.kt

@@ -0,0 +1,9 @@
+// TARGET_BACKEND: JVM_IR
+// IGNORE_BACKEND_K1: JVM_IR
+
+fun foo(x: String = "OK"): String = x
+
+fun box(): String {
+    val f: () -> String = ::foo
+    return f()
+}