[Demangle] replace std::string_view::substr which may throw

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: f198e0b5 ("[StringView] remove dropFront")

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D148959