Most of the complexity here is teaching SILGen how to handle closed-over direct
accesses to observing properties, since all of the getter/setter/willSet/didSet
members of the property are actually full closures when in a function body.
We generate correct but really terrible code here, since the setter captures the
willset/didset members themselves. I'm not worrying about the performance of
this construct though, functionality is what matters.
Swift SVN r13778
AnyFunctionRef is a universal function reference that can wrap all AST nodes
that represent functions and exposes a common interface to them. Use it in two
places in SIL where CapturingExpr was used previously.
AnyFunctionRef allows further simplifications in other places, but these will
be done separately.
Swift SVN r8239
