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/context.cpp

@@ -73,7 +73,7 @@ auto Context::VerifyOnFinish() const -> void {
   vtable_stack_.VerifyOnFinish();
   region_stack_.VerifyOnFinish();
   CARBON_CHECK(impl_lookup_stack_.empty());
-  CARBON_CHECK(return_type_inst_id_ == std::nullopt);
+  CARBON_CHECK(return_form_expr_ == std::nullopt);
 
 #ifndef NDEBUG
   if (auto verify = sem_ir_->Verify(); !verify.ok()) {