Move All the Gyb Support for Syntax out of Swift
Make libSyntax depend on swift-syntax: the new home for all
of this infrastructure. This greatly simplifies the addition and
amending of syntax nodes as only the swift-syntax paired with a
swift checkout will need to be changed. This is in contrast to
the existing build flow where a paired PR to both repos must be
made to change anything here.
Note that a paired PR may still be required if the legacy parser
needs to be adjusted in response to syntax nodes changing, but I
anticipate this to be a much more infrequent event now that
the C++ end of libSyntax is deprecated.
