swift234

[SIL] Let alloc_box return a single value.

And use project_box to get to the address value.
SILGen now generates a project_box for each alloc_box.
And IRGen re-uses the address value from the alloc_box if the operand of project_box is an alloc_box.
This lets the generated code be the same as before.

Other than that most changes of this (quite large) commit are straightforward.