Revert D148384 "[Demangle] replace use of llvm::StringView w/ std::string_view"

This reverts commit 3e559509 and e0c4ffa7.

This still breaks Windows builds.

In addition, `#include <llvm/ADT/StringViewExtras.h>` in
llvm/include/llvm/Demangle/ItaniumDemangle.h is a library layering violation
(LLVMDemangle is the lowest LLVM library and cannot depend on LLVMSupport).