Make SemIR::TypeType be an empty FacetType instruction (#7813)

The type `type` is now a `FacetType` inst with no constraints. This
brings the model implemented in the toolchain into better alignment with
the language design. The `SemIR::TypeType` struct remains as a scope for
holding the `TypeInstId`, `ConstantId`, and `TypeId` constants, but is
not an `InstKind` anymore.

The `TypeType` inst looks a lot like singletons, but there are many
`FacetType` insts so it doesn't quite fit that model. So we put it
alongside singletons with a fixed inst id but refer to it as a more
general "builtin" inst that is not a singleton.
`Namespace::PackageInstId` is similar, and we group it with `TypeType`
conceptually as another builtin instruction with a fixed id.

No conversion is needed anymore to use a `type` as a facet, since types
also have a `FacetType` type. This simplifies and removes a number of
helpers and branches throughout the code.

The `TypeType` inst is now part of the constant store, so we end up
printing it in the constants block in every test. But it's also named
`type` rather than `%type` to preserve the majority of existing
formatting behaviour, though this does look different from other
constants.

Assisted-by: Opus 5 was used to generate a first draft and validate the
refactoring. Though nearly everything non-trivial the tool wrote has
been modified or rewritten.

toolchain/check/handle_binding_pattern.cpp

@@ -613,7 +613,7 @@ auto HandleParseNode(Context& context,
   // compile time binding. This is popped when handling the
   // CompileTimeBindingPatternId.
   context.scope_stack().PushForSameRegion();
-  MakePeriodSelfFacetValue(context, node_id, GetEmptyFacetType(context));
+  MakePeriodSelfFacetValue(context, node_id, SemIR::TypeType::TypeId);
   context.node_stack().Push(
       node_id, SemIR::ElementIndex(context.binding_type_where_count()));
   return true;