[DiagnosticEngine] Don't account ignored diagnostics in Sema.NumSwift6Errors statistic

Since the diagnostic is not going to be emitted counting it in
`Swift6Errors` statistics is going to be confusing to the users.

Resolves: #79291
Resolves: rdar://145341605
(cherry picked from commit 664c119)

Author: xedin

lib/AST/DiagnosticEngine.cpp

@@ -432,7 +432,8 @@ InFlightDiagnostic::limitBehaviorUntilSwiftVersion(
     limitBehavior(limit);
   }
 
-  if (majorVersion == 6) {
+  // Record all of the diagnostics that are going to be emitted.
+  if (majorVersion == 6 && limit != DiagnosticBehavior::Ignore) {
     if (auto stats = Engine->statsReporter) {
       ++stats->getFrontendCounters().NumSwift6Errors;
     }