Drop Singleton from ErrorInst::SingletonInstId and similar (#5304)

We frequently want to operate on singletons. Per discussion, drop
`Singleton` to make the code shorter.

This started off as wanting to write `inst_id.is_error()`, but the
dependency relationship between ids.h and singleton_insts.h would
require some kind of delayed evaluation to allow the implementation to
remain in headers (which I suspect is helpful to have for inlining). I
could have added something like `IsErrorInst`, forward declared in ids.h
and defined in singleton_insts.h (which would always be included by
typed_insts.h), but the template approach felt like a decent balance
between (a) removing the boilerplate `::SingletonInstId`, (b)
understandability, (c) still visually mirroring if we immediately return
a singleton, and (d) flexibility for more than just `ErrorInst`. But TBH
I'd probably still have written `is_error()` if it didn't require
addressing the cross-header cycle.

Then I tried `SemIR::InstId::Is<SemIR::ErrorInst>`, which generally
worked with types but generated the complaint that it didn't shorten
*all* singleton uses. So pulling back on `::Is`, and instead just
dropping `Singleton`.

toolchain/check/handle_interface.cpp

@@ -52,9 +52,8 @@ static auto BuildInterfaceDecl(Context& context,
   auto decl_block_id = context.inst_block_stack().Pop();
 
   // Add the interface declaration.
-  auto interface_decl =
-      SemIR::InterfaceDecl{SemIR::TypeType::SingletonTypeId,
-                           SemIR::InterfaceId::None, decl_block_id};
+  auto interface_decl = SemIR::InterfaceDecl{
+      SemIR::TypeType::TypeId, SemIR::InterfaceId::None, decl_block_id};
   auto interface_decl_id = AddPlaceholderInst(context, node_id, interface_decl);
 
   SemIR::Interface interface_info = {name_context.MakeEntityWithParamsBase(