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/kt3429.kt

@@ -0,0 +1,17 @@
+open class Base {
+    open fun sayHello(): String{
+        return "fail"
+    }
+}
+
+trait Trait: Base {
+    override fun sayHello(): String {
+        return "OK"
+    }
+}
+
+class Derived(): Base(), Trait
+
+fun box() : String {
+    return Derived().sayHello()
+}