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/sem_ir/expr_info.cpp

@@ -93,21 +93,6 @@ static auto GetExprCategoryImpl(const File* ir, InstId inst_id)
             return ExprCategory::ReprInitializing;
           }
         }
-      } else if constexpr (std::same_as<TypedInstT, FormBinding>) {
-        auto form_id = ir->entity_names().Get(inst.entity_name_id).form_id;
-        auto form_inst = ir->insts().Get(form_id);
-        CARBON_KIND_SWITCH(form_inst) {
-          case InitForm::Kind:
-            // A `var` binding pattern produces a `ref` binding.
-          case RefForm::Kind:
-            return ExprCategory::DurableRef;
-          case ValueForm::Kind:
-            return ExprCategory::Value;
-          case ErrorInst::Kind:
-            return ExprCategory::Error;
-          default:
-            CARBON_FATAL("Unexpected kind for form inst {0}", form_inst);
-        }
       } else {
         static_assert(
             TypedInstT::Kind.expr_category().TryAsComputedCategory() !=