Fix incorrect diagnostic message for CONFLICTING_OVERLOADS

- the member and the container were mixed up
- don't output "defined in" part of the message, because we tell later where
  it's already defined in
- fix an instability of the diagnostic order in OverloadResolver by using
  LinkedHashSet

idea/testData/diagnosticMessage/conflictingOverloadsClass.kt

@@ -0,0 +1,7 @@
+// !DIAGNOSTICS_NUMBER: 2
+// !DIAGNOSTICS: CONFLICTING_OVERLOADS
+
+class conflictingOverloads {
+    fun lol(x: Int) = x
+    fun lol(y: Int) = y
+}