[Demangle] use std::string_view::data rather than &*std::string_view::begin

To fix expensive check builds that were failing when using MSVC's
std::string_view::iterator::operator*, I added a few expressions like
&*std::string_view::begin.  @nico pointed out that this is literally the
same thing and more clearly expressed as std::string_view::data.

Link: https://github.com/llvm/llvm-project/issues/63740

Reviewed By: #libc_abi, ldionne, philnik, MaskRay

Differential Revision: https://reviews.llvm.org/D154876