Avoid recalculating the field name length in the name comparison loop inside BSONObjBuilder::hasField().

The loop previously called `BSONElement::fieldName()`, which only returns a `const char*` pointing to the beginning of the field name. Converting this into an `std::string_view` for comparison required calling `strlen()` on the character pointer.
This is avoidable work, because `BSONElement::fieldNameStringData()` can already return an `std::string_view` with the already-established fieldname length.