[frontend] Add support for parsing ImmediateArgv.

These are not yet used, since the integrated frontend doesn’t yet support immediate mode, but they are ready for use once immediate mode is supported.

Swift SVN r11326

include/swift/Frontend/FrontendOptions.h

@@ -47,6 +47,9 @@ public:
   /// frontend invocation.
   std::string SerializedDiagnosticsPath;
 
+  /// Arguments which should be passed in immediate mode.
+  std::vector<std::string> ImmediateArgv;
+
   /// Indicates whether or not the frontend should print statistics upon
   /// termination.
   bool PrintStats = false;