[ADT] Add `StringRef::rsplit(StringRef Separator)`.

Summary: Add `StringRef::rsplit(StringRef Separator)` to achieve the function of getting the tail substring according to the separator. A typical usage is to get `data` in `std::basic_string::data`.

Reviewers: mehdi_amini, zturner, beanz, xbolva00, vsk

Reviewed By: zturner, xbolva00, vsk

Subscribers: vsk, xbolva00, llvm-commits, MTC

Differential Revision: https://reviews.llvm.org/D47406

llvm-svn: 334283