Refactor configurable constants in IR.

[otrho]

Currently the configurable constants are treated as regular consts in IR, and are post processed to find their address in the data section during ASMgen. While implementing the constdemotion pass in #4336 I accidentally, but very easily, corrupted their use by copying their values to the stack when they required demotion.

This broke the ASMgen post processing as it appeared that they were no longer used, and were essentially DCE'd.

Configurable constants should instead be treated as volatile memory regions rather than constants. They must always be backed by memory and must never be copied or combined as constant optimisers are wont to do.

They should have an instruction like get_cfg, which behaves much like get_local in that it represents a typed memory region and always returns a Pointer. #4336 had to add some hacks to mimic this behaviour, of making a constant value actually have a pointer type.

Placing them in a global memory manager as proposed by #4351 makes the most sense. But refactoring them out of being Values is non-trivial, as they are used by the const_eval module in sway-core as such quite extensively.