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/pattern.cpp

@@ -55,7 +55,7 @@ auto GetFirstBindingNameFromPatternId(const File& sem_ir, InstId pattern_id)
 
     // TODO: Look through struct patterns.
 
-    if (auto form_pattern = inst.TryAs<FormParamPattern>()) {
+    if (auto ref_pattern = inst.TryAs<RefParamPattern>()) {
       // TODO: This introduces a name, but we don't model it as a binding.
       return EntityNameId::None;
     }