Restructure ArgAndKind as a type-safe generic ID. (#7172)

The key changes here are:
- Relocating and renaming it to align with `IdKind` (and relocating
`ToRaw` and `FromRaw` to follow it).
- Adding a `Dispatch` method that provides a generic overload-based API
for expressing per-ID-kind dispatch, and rewriting existing code to use
it.

Note in particular that using overloads instead of switch cases makes it
possible to generically handle all specializations of a templated ID
type, e.g. `SomeIdType<T>` for all `T`. We have no such templated ID
types yet, but I'm introducing one in a follow-up PR that needs this
capability.

toolchain/sem_ir/expr_info.cpp

@@ -15,7 +15,7 @@
 namespace Carbon::SemIR {
 
 // Returns the InstId represented by an instruction operand.
-static auto AsAnyInstId(Inst::ArgAndKind arg) -> InstId {
+static auto AsAnyInstId(IdAndKind arg) -> InstId {
   if (auto inst_id = arg.TryAs<SemIR::InstId>()) {
     return *inst_id;
   }