[llvm-tblgen] - Stop using std::string in RecordKeeper.

RecordKeeper::getDef() is a hot place, it shows up in profiling
and it creates std::string instance for each search in RecordMap
though RecordKeeper::RecordMap can use StringRef as a key
instead to avoid that. Patch do that change.

Differential revision: https://reviews.llvm.org/D40170

llvm-svn: 318822