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/common.kt

@@ -0,0 +1,9 @@
+header class Foo {
+    fun f(s: String)
+
+    fun f(a: Any)
+}
+
+header fun g(s: String)
+
+header fun g(a: Any)