Adds per-builtin instructions, removing BuiltinInst

[jonmeow]

Adds per-builtin instructions, removing BuiltinInst. This collapses builtin_inst_kind.def into inst_kind.def so that we have a single place for all macro uses. I still want to remove BuiltinInstKind, but it's something I think is better separated from the BuiltinInst removal.

I'm collapsing the build targets ids and inst_kind into one because they both have links to builtin kind information now. It's hard to separate without a cycle. I'm using the typed_insts name because that seems like the actual most significant thing there, and more interesting relative to the inst target.

[jonmeow]

Depends on #4555