Converts TU-local `std::string` definitions for metadata field names into constexpr `StringData` values.
The variable names of these values were adjusted to use the `k` prefix for constants.
The metadata field name string values were previously stored in a `StringMap`, which is now converted into a `StringDataMap`. Also removes a now-unnecessary conversion from an `std::string` to `StringData`.
Finally, replaces an `std::unordered_map` with an `stdx::unordered_map`, which has much better memory locality.