swift234

[IRGen] Remove the AST walk that looks for local type decls.

Per the previous commit we are no longer using this. Minor save in
simplicity and maybe a bit of compilation time as well.

In the long run IRGen probably shouldn't be pulling information from the
AST at all; the SILModule should be able to tell it what types it needs
to emit information for. But this is still an improvement for now.

No functionality change (that was the previous commit).

Swift SVN r24840