Improve header/impl mismatch diagnostic messages

Try to report most mismatch errors on the 'impl' declaration. Only
report a mismatch error on the 'header' declaration if no error would be
otherwise reported on any 'impl' declaration in the compilation unit.
Also render declaration kind in the message

 #KT-18447 In Progress

compiler/testData/multiplatform/missingOverload/jvm.kt

@@ -0,0 +1,5 @@
+impl class Foo {
+    impl fun f(s: String) {}
+}
+
+impl fun g(a: Any) {}