[Frontend] Allow additional files for diagnostic verifier

This change adds a frontend flag, -verify-additional-file, which can be used to pass extra files directly to the diagnostic verifier. These files are not otherwise considered to be Swift source files; they are not compiled or even properly parsed.

This feature can be used to verify diagnostics emitted in non-source files, such as in module interfaces or header files.

include/swift/Frontend/Frontend.h

@@ -560,8 +560,11 @@ private:
   bool setUpInputs();
   bool setUpASTContextIfNeeded();
   void setupStatsReporter();
-  void setupDiagnosticVerifierIfNeeded();
   void setupDependencyTrackerIfNeeded();
+
+  /// \return false if successsful, true on error.
+  bool setupDiagnosticVerifierIfNeeded();
+
   Optional<unsigned> setUpCodeCompletionBuffer();
 
   /// Find a buffer for a given input file and ensure it is recorded in