[StringView] remove popFront

Towards converting our use of llvm::StringView to std::string_view,
remove a method that std::string_view doesn't have.

llvm::StringView::popFront is similar to std::string_view::remove_prefix
but with a reference to std::string_view::front taken first.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D148363