[libcxxabi][demangle] create helper for std::string_view::starts_with

Add C++ 20 style starts_with to replace StringView::startsWith in
LLVMDemangle. Due to library layering (LLVMSupport depends on
LLVMDemangle), we add the utility header under llvm/Demangle, instead of
llvm/ADT or llvm/Support.

Modify this in libcxxabi, then copy this over to llvm.

Reviewed By: MaskRay, #libc_abi, phosek

Differential Revision: https://reviews.llvm.org/D148556