Initial support for return forms (#6556)

The main changes here are:
- Introducing `InitForm` and `RefForm` to represent initializing and
reference forms (the two return forms currently supported by the
parser).
- Introducing the `FormType` singleton inst to represent their type
(i.e. `Core.Form`).
- Emitting an inst representing a function's declared return form as
part of handling the function signature.

The return form inst is currently ignored. Subsequent PRs will expose it
in `SemIR::Function` and use it to determine the form of call
expressions.

toolchain/check/function.cpp

@@ -139,8 +139,6 @@ auto CheckFunctionDefinitionSignature(Context& context,
   if (function.return_type_inst_id.has_value()) {
     CheckFunctionReturnType(context, SemIR::LocId(function.return_type_inst_id),
                             function, SemIR::SpecificId::None);
-    // Don't re-check the return type below.
-    params_to_complete.consume_back();
   }
 
   // Check the parameter types are complete.