[RustDemangle] remove StringView::dropFront

Toward the goal of replacing llvm::StringView with std::string_view,
first replacing users of llvm::StringView::dropFront, this case in the
Rust demangling scheme seemed worth its own commit+review.

Reviewed By: erichkeane, MaskRay

Differential Revision: https://reviews.llvm.org/D148272