[lldb][NFCI] Minor cleanup of default OptionValue::GetSubValue implementation

This does 2 things:
- Corrects a minor typo (`value subvalue` -> `valid subvalue`)
- Removes an unnecessary instance of `str().c_str()` (creating a
  temporary std::string from a StringRef just to get a valid
  null-terminated string).

Differential Revision: https://reviews.llvm.org/D154387