Refactor ImplementationBodyCodegen.getTraitImplementations()

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

compiler/testData/codegen/bytecodeText/traitImplGeneratedOnce.kt

@@ -0,0 +1,15 @@
+trait A {
+    fun foo() = 42
+}
+
+open class B : A
+
+open class C : B()
+
+class D : C()
+
+// Implementation of foo() should only be generated into B
+// 1 INVOKESTATIC A\$\$TImpl.foo
+
+// Only two declarations should be present: in A and B
+// 2 foo\(\)I