Define dummy as unused error message id

I am exasperated how over-engineered and badly designed the error message architecture is.
Error messages used to be simple strings but are now a complete mess.

Latest installment: Error message ids are an enum. So if a message is no longer needed,
you can drop the enum value, right? Wrong! The error message tests rely on the actual
ordinal number printed, so they fail if that number changes.

Author: odersky

compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.scala

@@ -91,6 +91,7 @@ enum ErrorMessageID extends java.lang.Enum[ErrorMessageID] {
     ExpectedTopLevelDefID,
     AnonymousFunctionMissingParamTypeID,
     SuperCallsNotAllowedInlineableID,
+    UNUSED1,  // not used anymore, but left so that error numbers stay the same
     WildcardOnTypeArgumentNotAllowedOnNewID,
     FunctionTypeNeedsNonEmptyParameterListID,
     WrongNumberOfParametersID,