Make Slot a Union type

[Liozou]

Closes #27202

I'm not familiar with these files, so it would be good if someone checked I haven't broken anything by moving Slot from the C builtins to boot.jl

[vtjnash]

Nice work! I haven't reviewed in much detail, but I'm amazed how small this is. Very recently though, TypedSlot became unused (outside of a brief appearance in the IR between inlining and optimization in Core.Compiler). We can still merge this, but if you're able to completely remove all references to it outside of base/compiler/optimize.jl and base/compiler/ssair/slot2ssa.jl, that would be even more epic! :)

[Liozou]

On master TypedSlot seems to be still used in boot.jl and compiler/utilities.jl so I'm afraid to break everything if I replace Slot by SlotNumber almost everywhere... Is this removal of TypedSlot on a specific branch?

[Liozou]

Actually, forget about boot.jl, that's not threatening.
I forgot to deal with lines 445 and 449 of boot.jl, but to say the truth I'm not sure to understand what they do. Anyway, should Slot be removed from them?

[vtjnash]

It's essentially executable documentation at

julia/base/boot.jl

Line 443 in db158b8

module IR

The work hasn't yet been done to detangle and remove the references to TypedSlot (it's only just recently become feasible).

[Liozou]

OK, in that case I'm going to have a look at what can be trimmed

[Liozou]

I made a scan of all the uses of Slot in the code and, apart from optimize.jl, it also mainly appears in typelattice.jl and abstractinterpretation.jl. I suppose you would like to replace it with SlotNumber as much as possible there, but I have no idea whether that would be correct in these cases since I don't know this part of the compiler at all.
As for TypedSlot itself, I remarked that it was used to define slot_id twice: once in utilities.jl and the other in ssair/slot2ssa.jl. Should one of the two be removed?

[vtjnash]

It's referenced (but will never actually appear) in abstractinterpretation.jl. It might appear in the typelattice, but you can define TypedSlot in that code (in Core.Compiler, and not a subtype of Slot) and handle it in the few cases where it is observed to appear.

[Liozou]

TypedSlot never explicitly appears in abstractinterpretation.jl, only Slot does and I don't know whether it can or cannot contain a TypedSlot value. This is kind of my issue whenever Slot is used: I never know whether it is safe to replace it by SlotNumber or not.

[aviatesk]

Will be replaced by #48693.