swift234

[SIL] Let alloc_stack return a single value.

Having a separate address and container value returned from alloc_stack is not really needed in SIL.
Even if they differ we have both addresses available during IRGen, because a dealloc_stack is always dominated by the corresponding alloc_stack in the same function.

Although this commit quite large, most changes are trivial. The largest non-trivial change is in IRGenSIL.

This commit is a NFC regarding the generated code. Even the generated SIL is the same (except removed #0, #1 and @local_storage).