[Syntax] Tablegen Sequence classes. NFC

[Syntax] Tablegen Sequence classes. NFC

Similar to the previous patch, this doesn't convert *all* the classes that
could be converted. It also doesn't enforce any new invariants etc.

It *does* include some data we don't use yet: specific token types that are
allowed and optional/required status of sequence items. (Similar to Dmitri's
prototype). I think these are easier to add as we go than later, and serve
a useful documentation purpose.

Differential Revision: https://reviews.llvm.org/D90659