Reimplement getToken and SplitString as "StringRef helper functions"

Reimplement getToken and SplitString as "StringRef helper functions"

- getToken is modeled after StringRef::split but it can split on multiple
  separator chars and skips leading seperators.
- SplitString is a StringRef::split variant for more than 2 elements with the
  same behaviour as getToken.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@93161 91177308-0d34-0410-b5e6-96231b3b80d8