Fix mapGetterSignature/mapSetterSignature

In case of TRAIT_IMPL the incorrect this was written. Reuse the code from the
common mapSignature

 #KT-3413 Fixed

compiler/testData/codegen/box/traits/withRequiredPropertyViaBridge.kt

@@ -0,0 +1,14 @@
+open class Base
+
+trait Trait : Base {
+    private val value : String
+        get() = "OK"
+        
+    fun toString() = object {
+        fun foo() = value
+    }.foo()
+}
+
+class Derived : Trait, Base()
+
+fun box() = "${Derived()}"