Codegen for callable reference expressions

#KT-1183 In Progress

compiler/testData/codegen/box/callableReference/topLevelFromTopLevelUnitOneStringArg.kt

@@ -0,0 +1,11 @@
+var result = "Fail"
+
+fun foo(newResult: String) {
+    result = newResult
+}
+
+fun box(): String {
+    val x = ::foo
+    x("OK")
+    return result
+}