[StringView] remove consumeFront

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

This could be moved to the nascent llvm/ADT/StringViewExtras.h, but the
use is highly localized to one TU. Move this to be a static function
there.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D148375