[Diagnostics] Add InFlightDiagnostic::warnUntilSwiftVersion to limit the

diagnostic behavior to a warning until the specified language version.

This helper can be used to stage in fixes for stricter diagnostics
as warnings until the next major language version.

lib/Frontend/Frontend.cpp

@@ -451,6 +451,8 @@ void CompilerInstance::setUpDiagnosticOptions() {
   }
   Diagnostics.setDiagnosticDocumentationPath(
       Invocation.getDiagnosticOptions().DiagnosticDocumentationPath);
+  Diagnostics.setLanguageVersion(
+      Invocation.getLangOptions().EffectiveLanguageVersion);
   if (!Invocation.getDiagnosticOptions().LocalizationCode.empty()) {
     Diagnostics.setLocalization(
         Invocation.getDiagnosticOptions().LocalizationCode,