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/sem_ir/stringify.cpp

@@ -706,10 +706,9 @@ auto StringifySpecific(const File& sem_ir, SpecificId specific_id)
       // TODO: This duplicates work done in StringifyInst for ClassType.
       const auto& class_info = sem_ir.classes().Get(class_decl.class_id);
       if (auto literal_info = NumericTypeLiteralInfo::ForType(
-              sem_ir,
-              SemIR::ClassType{.type_id = SemIR::TypeType::SingletonTypeId,
-                               .class_id = class_decl.class_id,
-                               .specific_id = specific_id});
+              sem_ir, SemIR::ClassType{.type_id = SemIR::TypeType::TypeId,
+                                       .class_id = class_decl.class_id,
+                                       .specific_id = specific_id});
           literal_info.is_valid()) {
         RawStringOstream out;
         literal_info.PrintLiteral(sem_ir, out);