swift234

Various cleanups for the initialization logic:
 - Use virtual dispatch to localize some predicates instead
   of having special cases for LetValue inits in global places.
 - Improve 'const'ness of methods.
 - Introduce a common "KnownAddressInitialization" class to centralize
   the notion of an initialization corresponding to a specific physical
   address that is known up front.  Consolidate dupes of this concept into
   uses of it.

NFC.



Swift SVN r27462