[StringView] remove dropFront

Towards converting our use of llvm::StringView to std::string_view,
remove a method that std::string_view doesn't have.
llvm::StringView::dropFront is semantically similar to
std::string_view::substr but with the input clamped to the size. No code
was relying on clamping other than the rust demangler, which I fixed in
https://reviews.llvm.org/D148272.  Removing this method makes it easier
to switch over code later.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D148348