An optimization introduced in SERVER-70553 attempts avoid re-reading a file from disk if we need to reinterpret the source as a module. Since the code stores the data in a standard JSString, it will corrupt the input UTF8 data and prevent accurate round-tripping of some characters. The fix is to store the data in a wide UCString (similar to how we store it when reading from the file in the ModuleLoader::fileAsString method)