swift234

This patch does a number of conflated things:
1) Implement sema synthesization of the getter/setter & storage for @lazy properties.
2) Rework has property synthesization works so that the synthesizers call typeCheckDecl
  on the stuff they create, instead of having the callers do it.
3) Refactor the synthesized setter portion of addAccessorsToStoredVar out into a
   synthesizeTrivialSetter function to mirror how it handles getters.
4) Add a new addMemberToContext helper to handle injection of decls into their parent
   declcontext, given that we don't have a proper model for IterableDeclContext's.
5) Use isClassOrClassExtensionContext() a bit more
6) Adjust the synthesized member-wise init method of a struct to work with @lazy 
   initialized fields.

Codegen is working in basic cases now, but there is still unfinished business.


Swift SVN r17904