Refactor ImplementationBodyCodegen.getTraitImplementations()

Make it static, extract a method which finds an implementation in a trait for a
fake override

compiler/testData/codegen/box/traits/withRequired/genericProperty.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
+}