Make the argument-label diagnostics slightly more robust.

If we're matching the trailing closure at the end but haven't seen any
mismatches yet, consider the trailing closure to be a mismatch.
This function probably needs to be rewritten in terms of
matchCallArguments(), because it is performing an incorrect
approximation of label matching that doesn't work for multiple
trailing closures.

Author: DougGregor

lib/Sema/MiscDiagnostics.cpp

@@ -1798,7 +1798,8 @@ bool swift::diagnoseArgumentLabelError(ASTContext &ctx,
       newName = newNames[i];
 
     if (oldName == newName ||
-        (argList.hasTrailingClosure && i == argList.args.size()-1))
+        (argList.hasTrailingClosure && i == argList.args.size()-1 &&
+         (numMissing > 0 || numExtra > 0 || numWrong > 0)))
       continue;
 
     if (oldName.empty()) {