[ORC-RT] Drop __orc_rt::string_view now that we have c++17.

b1356504e63 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.
GitOrigin-RevId: 1dcff823db9f4dd5c300507f679cdb2494287442