swift234

SIL: Add a new alloc_global instruction

If a global variable in a module we are compiling has a type containing
a resilient value type from a different module, we don't know the size
at compile time, so we cannot allocate storage for the global statically.

Instead, we will use a buffer, just like alloc_stack does for archetypes
and resilient value types.

This adds a new SIL instruction but does not yet make use of it.