llvm/Demangle copies the implementation from libcxxabi/src/demangle/.
libcxxabi/ cannot use potentially-throwing std::string_view::substr, so
change llvm/Demangle to avoid these function calls.
I ran into linkage failures stemming from the usage of
std::string_view::substr. substr does a bounds check and may throw.
Fixes: f198e0b594aa ("[StringView] remove dropFront")