Split out the StreamEndStop helper into its own header

src/catch2/catch_message.hpp

@@ -9,8 +9,8 @@
 #define CATCH_MESSAGE_HPP_INCLUDED
 
 #include <catch2/internal/catch_result_type.hpp>
-#include <catch2/internal/catch_common.hpp>
 #include <catch2/internal/catch_stream.hpp>
+#include <catch2/internal/catch_stream_end_stop.hpp>
 #include <catch2/internal/catch_message_info.hpp>
 #include <catch2/interfaces/catch_interfaces_capture.hpp>
 #include <catch2/catch_tostring.hpp>
@@ -20,6 +20,8 @@
 
 namespace Catch {
 
+    struct SourceLineInfo;
+
     struct MessageStream {
 
         template<typename T>