[clang] Use try_emplace instead of insert when getting new identifier

This is both less verbose and slightly faster, according to:

https://llvm-compile-time-tracker.com/compare.php?from=d9ab3e82f30d646deff054230b0c742704a1cf26&to=73405077ad913f634797ffc7a7bbb110ac9cae99&stat=instructions:u

No functional change intended :-)