[lldb][NFCI] Remove use of ConstString from StructuredDataDarwinLog static functions

The strings "DarwinLog" and "log" probably do not need to be in the
ConstString StringPool. We still create ConstStrings from them in some
places (for now) but that's because we don't have an implicit
constructor to convert a StringRef to a ConstString.

Differential Revision: https://reviews.llvm.org/D151599