[ADT] Fix a confusing interface spec and some annoying peculiarities

[ADT] Fix a confusing interface spec and some annoying peculiarities
with the StringRef::split method when used with a MaxSplit argument
other than '-1' (which nobody really does today, but which should
actually work).

The spec claimed both to split up to MaxSplit times, but also to append
<= MaxSplit strings to the vector. One of these doesn't make sense.
Given the name "MaxSplit", let's go with it being a max over how many
*splits* occur, which means the max on how many strings get appended is
MaxSplit+1. I'm not actually sure the implementation correctly provided
this logic either, as it used a really opaque loop structure.

The implementation was also playing weird games with nullptr in the data
field to try to rely on a totally opaque hidden property of the split
method that returns a pair. Nasty IMO.

Replace all of this with what is (IMO) simpler code that doesn't use the
pair returning split method, and instead just finds each separator and
appends directly. I think this is a lot easier to read, and it most
definitely matches the spec. Added some tests that exercise the corner
cases around StringRef() and StringRef("") that all now pass.

I'll start using this in code in the next commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247249 91177308-0d34-0410-b5e6-96231b3b80d8