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/cpp/export.cpp

@@ -332,8 +332,7 @@ static auto ExportGenericBindings(Context& context, SemIR::LocId loc_id,
     CARBON_CHECK(param_ident, "non-identifier param name {0}",
                  entity_name.name_id);
 
-    if (symbolic_binding.type_id != SemIR::TypeType::TypeId &&
-        !context.types().Is<SemIR::FacetType>(symbolic_binding.type_id)) {
+    if (!context.types().Is<SemIR::FacetType>(symbolic_binding.type_id)) {
       context.TODO(loc_id, "binding maps to a non-type template parameter");
       return nullptr;
     }