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/node_stack.h

@@ -426,6 +426,7 @@ class NodeStack {
       case Parse::NodeKind::IfExprIf:
       case Parse::NodeKind::ImplicitParamList:
       case Parse::NodeKind::WhileConditionStart:
+      case Parse::NodeKind::ReturnType:
         return Id::KindFor<SemIR::InstBlockId>();
       case Parse::NodeKind::FunctionDefinitionStart:
       case Parse::NodeKind::BuiltinFunctionDefinitionStart:
@@ -446,8 +447,6 @@ class NodeStack {
       case Parse::NodeKind::DefaultLibrary:
       case Parse::NodeKind::LibraryName:
         return Id::KindFor<SemIR::LibraryNameId>();
-      case Parse::NodeKind::ReturnType:
-        return Id::KindFor<SemIR::InstId>();
       case Parse::NodeKind::AssociatedConstantInitializer:
       case Parse::NodeKind::AssociatedConstantIntroducer:
       case Parse::NodeKind::BuiltinName: