swift234

Split SIL emission for builtin functions into its own file.

NFC for now, but I've also added the infrastructure to allow
"early emission", i.e. emission directly from the original
RValueSource, which can be useful either as an optimization
(e.g. for Builtin.initialize) or a requirement for particularly
hacky SIL intrinsics should we need them (and I'm thinking of
needing one).

Swift SVN r23953