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/check/deduce.cpp

@@ -110,7 +110,7 @@ class DeductionWorklist {
   }
 
   // Adds a (param, arg) pair for an instruction argument, given its kind.
-  auto AddInstArg(SemIR::Inst::ArgAndKind param, int32_t arg) -> void {
+  auto AddInstArg(SemIR::IdAndKind param, int32_t arg) -> void {
     CARBON_KIND_SWITCH(param) {
       case SemIR::IdKind::None:
       case SemIR::IdKind::For<SemIR::ClassId>: