Rename Builtin to BuiltinInst, particularly to get BuiltinInstKind

[jonmeow]

I'm trying to increase the distinction between BuiltinKind and BuiltinFunctionKind. BuiltinKind is for instructions, BuiltinFunctionKind is for function definitions. To get to this point, I'm doing a few changes:

• BuiltinKind -> BuiltinInstKind
  • builtin_kind. -> builtin_inst_kind.: filename consistency
• Builtin -> BuiltinInst: mainly for consistency with the above
  • Builtin::builtin_kind -> BuiltinInst::builtin_inst_kind: somewhat repetitive but seems like a consistent edit
• Function::builtin_kind -> Function::builtin_function_kind: seems a useful distinction

I'm leaving alone things like (and mentioning in case there's a desire for more renames):

• InstId::BuiltinError, InstId::ForBuiltin: these I think are more apparent because they're directly associated with Inst.
• GetBuiltinICmpPredicate in lowering: maybe builtin function handling should be in its own file, but these local names don't feel problematic to me.
• GetBuiltinType, BuildBuiltinValueRepr, PerformBuiltinIntComparison: similar to the above, names don't feel too problematic