SERVER-8130 Fix invalid access in textSearchEnabled check

Bug introduced in fts_index.cpp, FTSIndexPlugin::adjustIndexSpec()
(not changed by diff):

 69|            StringData desc  = cc().desc();

Previously, Client::desc() returned std::string, so desc._data
became a dangling pointer to destroyed temporary cc().desc().
Resolved by changing return type of Client::desc() to StringData.