swift234

Implement: <rdar://problem/16181314> don't require immediate initialization of 'let' values
... now that we have an exquisitely shaved yak.

This provides a simple and uniform model for "let" constants: they are always either
immediately initialized in their declaration, or they are initialized dynamically
exactly once before any use.  

This is a simple generalization of our current model for initializers, but enables
the use of let constants in more cases in local context, e.g. patterns like this:

   let x : SomeThing

   if condition {
     x = foo()
   } else {
     x = bar()
   }
   use(x)

Previously this would have to be declared a "var" for no good reason: the value is
only ever initialized, never actually mutated.

The implementation of this is reasonably straight-forward now that the infrastructure
is in place: Sema treats 'let' constants as "settable" if they lack an initializer
(either in the declaration or in a non-PBD binding).  This exposes them as an lvalue
at the AST level.  SILGen then lowers these things to an alloc_stack, and DI enforces
the "initialization only" requirement that it already enforces for uninitialized 'let'
properties in structs/classes.



Swift SVN r23916