[reflection] Extract out ReflectionInfo from TypeRefBuilder -> ReflectionInfo.

The reason why I am doing this is so that I can create an adaptor class
(templated on Runtime) for reading protocol info from ReflectionInfo without
having to make ReflectionInfo itself generic. If ReflectionInfo becomes generic
on Runtime, it will cause a cascading need to mark classes in Reflection as
generic as well.

include/swift/Reflection/MetadataSourceBuilder.h

@@ -18,6 +18,8 @@
 #define SWIFT_REFLECTION_METADATASOURCEBUILDER_H
 
 #include "swift/Reflection/MetadataSource.h"
+#include <memory>
+#include <vector>
 
 namespace swift {
 namespace reflection {