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/eval_inst.cpp

@@ -124,7 +124,7 @@ auto EvalConstantInst(Context& context, SemIR::ClassDecl inst)
 
   // A non-generic class declaration evaluates to the class type.
   return ConstantEvalResult::NewSamePhase(
-      SemIR::ClassType{.type_id = SemIR::TypeType::SingletonTypeId,
+      SemIR::ClassType{.type_id = SemIR::TypeType::TypeId,
                        .class_id = inst.class_id,
                        .specific_id = SemIR::SpecificId::None});
 }
@@ -311,8 +311,7 @@ auto EvalConstantInst(Context& context, SemIR::NameRef inst)
 
 auto EvalConstantInst(Context& context, SemIR::InstId inst_id,
                       SemIR::RequireCompleteType inst) -> ConstantEvalResult {
-  auto witness_type_id =
-      GetSingletonType(context, SemIR::WitnessType::SingletonInstId);
+  auto witness_type_id = GetSingletonType(context, SemIR::WitnessType::InstId);
 
   // If the type is a concrete constant, require it to be complete now.
   auto complete_type_id =