Reland: [Demangle] replace use of llvm::StringView w/ std::string_view

This reverts commit d81cdb49.

This refactoring was waiting on converting LLVM to C++17.

Leave StringView.h and cleanup around for subsequent cleanup.

Additional fixes for missing std::string_view conversions for MSVC.

Reviewed By: MaskRay, DavidSpickett, ayzhao

Differential Revision: https://reviews.llvm.org/D148546