swift234

Sigificantly replumb how SILGen generates mark_uninitialized instructions, generating
them in a more consistent and principled way.  Two changes here: MUI is generated
when a vardecl is emitted, not as a separate "MarkPatternUninitialized" pass.  Second,
when generating a MUI for self parameters with a temporary alloc_stack (due to the
possibility of superclass remapping of self) emit the MUI on the allocation itself,
not on the incoming argument.  This is a lot more consistent (dissolving a bunch of 
hacks in DI).

In terms of behavior changes, this only changes the raw sil generated by SILGen and
consumed by DI, so there is no user-visible change.  This simply unblocks future work.



Swift SVN r23823