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/withRequiredGenericProperty.kt

@@ -0,0 +1,19 @@
+abstract class Base<T> {
+    abstract var s: T
+}
+
+trait Trait<T> : Base<T> {
+    var value : T
+        get() = s
+        set(value) { s = value }
+}
+
+class Derived : Trait<String>, Base<String>() {
+    override var s = "Fail"
+}
+
+fun box(): String {
+    val d = Derived()
+    d.value = "OK"
+    return d.value
+}

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

@@ -0,0 +1,17 @@
+open class Base {
+    var s = "Fail"
+}
+
+trait Trait : Base {
+    var value : String
+        get() = s
+        set(value) { s = value }
+}
+
+class Derived : Trait, Base()
+
+fun box(): String {
+    val d = Derived()
+    d.value = "OK"
+    return d.value
+}

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()}"