Drop most of InitializationSequence::SequenceKind's values. They didn't really...

Drop most of InitializationSequence::SequenceKind's values. They didn't really contain any information that the step array didn't contain too. This makes debugging dumps a bit less informative, but probably not significantly so. The advantage is that the redundancy is gone, so the code is easier to understand.
ReferenceBinding is still there, because it is used in some unclear code.

llvm-svn: 132667