remove Demangle/StringView.h

Now that we've converted libcxxabi and llvm Demangle to use
std::string_view, this code no longer has any users. Bye bye!

Reviewed By: #libc_abi, phosek, MaskRay

Differential Revision: https://reviews.llvm.org/D148387

GitOrigin-RevId: 217709cbae34ec59ed366f6d77b5d612d2018245
2 files changed