Replace form insts with actions (#7100)

See
[here](https://docs.google.com/document/d/1rWcueFwIfZox6GKVGxiUG4cBzjrZ6djXiIDGyJDtrE4/edit?tab=t.0)
for the design doc.

This also removes the default value of the `result_type_inst_id`
parameter of `HandleAction`, moves it before the action in the parameter
list, and documents it. This solves two problems:
- The default made it easy to forget, leading to unnecessary
`TypeOfInst` instructions.
- When it was present, putting it after the fairly "bulky" action
argument tended to make the callsite harder to read.

toolchain/check/eval.cpp

@@ -2369,7 +2369,9 @@ static auto TryEvalTypedInst(EvalContext& eval_context, SemIR::InstId inst_id,
     if constexpr (ConstantKind == SemIR::InstConstantKind::Always ||
                   ConstantKind == SemIR::InstConstantKind::WheneverPossible) {
       return MakeConstantResult(eval_context.context(), inst, phase);
-    } else if constexpr (ConstantKind == SemIR::InstConstantKind::InstAction) {
+    } else if constexpr (ConstantKind ==
+                             SemIR::InstConstantKind::ConstantInstAction ||
+                         ConstantKind == SemIR::InstConstantKind::InstAction) {
       auto result_inst_id = PerformDelayedAction(
           eval_context.context(), SemIR::LocId(inst_id), inst.As<InstT>());
       if (result_inst_id.has_value()) {