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/lower/function_context.cpp

@@ -175,8 +175,10 @@ auto FunctionContext::IsConstant(SemIR::InstId inst_id) -> bool {
 }
 
 auto FunctionContext::GetValue(SemIR::InstId inst_id) -> llvm::Value* {
-  // All builtins are types, with the same empty lowered value.
-  if (SemIR::IsSingletonInstId(inst_id)) {
+  // Singletons are types, as is the builtin TypeType, with the same empty
+  // lowered value.
+  if (SemIR::IsSingletonInstId(inst_id) ||
+      inst_id == SemIR::TypeType::TypeInstId) {
     return GetTypeAsValue();
   }