b1356504e6 enabled the use of c++17 features in LLVM, which means that we can
drop the ORC runtime's placeholder string_view implemention in favor of
std::string_view.
This will simplify integration of this code into LLVM -- The
Simple-Packed-Serialization code can be copied near-verbatim, but
WrapperFunctionResult will require more adaptation.
Lang Hames