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.
Adds construction from std::string, an ostream &operator<< and std::hash
specialization. Also adds unit tests for each of these operations, as well as
tests for copy construction and assignment.
These new operations will be used in upcoming macho_platform patches.
Lang Hames