swift234

Fix <rdar://problem/16871284> silgen crashes on weak capture

This reworks handling of weak/unowned values to represent them as logical 
lvalues instead of as physical lvalues.  Representing them as physical
lvalues breaks a lot of assumptions throughout silgen because (while they
have the same in-memory representation) weak pointers and optional have
different dynamic behavior.

This will hopefully make weak pointers generally more reliable than they
have been.  One causualty of this is that the "copy_addr" peephole is defeated
for the case that you copy a weak lvalue to another weak lvalue, but correctness
needs to win, and this can be added back later when it becomes a priority.



Swift SVN r17842