[ADT] Add a single-character version of the small vector split routine

[ADT] Add a single-character version of the small vector split routine
on StringRef. Finding and splitting on a single character is
substantially faster than doing it on even a single character StringRef
-- we immediately get to a *very* tuned memchr call this way.

Even nicer, we get to this even in a debug build, shaving 18% off the
runtime of TripleTest.Normalization, helping PR23676 some more.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247244 91177308-0d34-0410-b5e6-96231b3b80d8