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/thunk.cpp

@@ -63,7 +63,6 @@ static auto CloneBindingPattern(Context& context, SemIR::InstId pattern_id,
   auto entity_name = context.entity_names().Get(pattern.entity_name_id);
   CARBON_CHECK((pattern.kind == SemIR::SymbolicBindingPattern::Kind) ==
                entity_name.bind_index().has_value());
-  CARBON_CHECK(pattern.kind != SemIR::FormBindingPattern::Kind);
   // Get the transformed type of the binding.
   if (new_pattern_type_id == SemIR::ErrorInst::TypeId) {
     return SemIR::ErrorInst::InstId;
@@ -80,8 +79,8 @@ static auto CloneBindingPattern(Context& context, SemIR::InstId pattern_id,
   if (pattern.kind == SemIR::WrapperBindingPattern::Kind) {
     auto subpattern = context.insts().GetAs<SemIR::AnyLeafParamPattern>(
         pattern.subpattern_id);
-    if (subpattern.kind == SemIR::FormParamPattern::Kind) {
-      context.TODO(pattern_id, "Support for cloning form bindings");
+    if (subpattern.kind == SemIR::FormParamPatternAction::Kind) {
+      context.TODO(pattern_id, "Support for cloning generic form bindings");
       return SemIR::ErrorInst::InstId;
     }
     pattern.subpattern_id = RebuildPatternInst<SemIR::AnyLeafParamPattern>(