[llvm] [ADT] Add a range/iterator-based Split()

Add a llvm::Split() implementation that can be used via range-for loop,
e.g.:

    for (StringRef x : llvm::Split("foo,bar,baz", ','))
      ...

The implementation uses an additional SplittingIterator class that
uses StringRef::split() internally.

Differential Revision: https://reviews.llvm.org/D110496