\brief Writes the block containing the serialized form of the source manager. TODO: We should probably use an on-disk hash table (stored in a blob), indexed based on the file name, so that we only create entries for files that we actually need. In the common case (no errors), we probably won't have to create file entries for any of the files in the AST.
source not stored for this graph (policy: none)
nothing calls this directly
no test coverage detected