[lldb] Avoid unnecessary strlen of mangled names in ConstString (NFC) (#197995)
C++ mangled names are known to be quite long at times. This change makes
use of available length data, instead of using the `StringRef(const char
*)` constructor which calls `strlen`.
The main detail is to replace `selectPool(llvm::StringRef(raw))` with a
call to `selectPool` using a readily available StringRef.