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/super.kt

@@ -0,0 +1,18 @@
+open class Base {
+    open fun foo() : String {
+      return "fail"
+    }
+}
+
+trait Derived : Base {
+    override fun foo() : String {
+        //super.foo()
+        return "OK"
+    }
+}
+
+class DerivedImpl : Derived, Base()
+
+fun box(): String {
+    return DerivedImpl().foo()
+}