swift234

IRGen: Lowering of alloc_global and global_addr for non-fixed-size types

Allocate and project the buffer, respectively, adding the
necessary indirection required to handle the size not being
known until runtime.

For now we don't emit alloc_global instructions anywhere;
an upcoming change will add that at the SIL level.

Also I suspect debug info needs some work to handle the
extra indirection, I'll look into this soon.