Improve comments on conversion from type value to type id (#4885)

The GetTypeIdForTypeConstant() and GetTypeIdForTypeInst() methods
convert from a type value to the type id of that value. The comments say
it can only be done for values of type `type` but it can be done for any
type value, which are either `TypeType` (aka `type`) or `FacetType`.

For example, for a `FacetValue` instruction as follows:
```
inst116: {kind: FacetValue, arg0: inst46, arg1: inst59, type: type(inst52)}
  - type type(inst52): Generic(GenericParam); {kind: FacetType, arg0: facet_type2, type: type(TypeType)}
  - value template_constant(inst117): {kind: FacetValue, arg0: inst46, arg1: inst59, type: type(inst52)}
```
The GetTypeIdForTypeInst() method will give the type id of the type
value (the type _in_ the FacetValue instruction) such as:
```
type(inst117): ImplsGeneric as Generic(GenericParam); {kind: FacetValue, arg0: inst46, arg1: inst59, type: type(inst52)}
```

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/check/context.cpp

@@ -838,8 +838,7 @@ auto Context::GetTypeIdForTypeConstant(SemIR::ConstantId constant_id)
                "Canonicalizing non-constant type: {0}", constant_id);
   auto type_id =
       insts().Get(constant_values().GetInstId(constant_id)).type_id();
-  // TODO: For now, we allow values of facet type to be used as types.
-  CARBON_CHECK(IsFacetType(type_id) ||
+  CARBON_CHECK(type_id == SemIR::TypeType::SingletonTypeId ||
                    constant_id == SemIR::ErrorInst::SingletonConstantId,
                "Forming type ID for non-type constant of type {0}",
                types().GetAsInst(type_id));

toolchain/check/context.h

@@ -294,11 +294,22 @@ class Context {
     return scope_stack().GetCurrentScopeAs<InstT>(sem_ir());
   }
 
-  // Returns the type ID for a constant of type `type`.
+  // Returns the type ID for a constant that is a type value, i.e. it is a value
+  // of type `TypeType`.
+  //
+  // Facet values are of the same typishness as types, but are not themselves
+  // types, so they can not be passed here. They should be converted to a type
+  // through an `as type` conversion, that is, to a value of type `TypeType`.
   auto GetTypeIdForTypeConstant(SemIR::ConstantId constant_id) -> SemIR::TypeId;
 
-  // Returns the type ID for an instruction whose constant value is of type
-  // `type`.
+  // Returns the type ID for an instruction whose constant value is a type
+  // value, i.e. it is a value of type `TypeType`.
+  //
+  // Instructions whose values are facet values (see `FacetValue`) produce a
+  // value of the same typishness as types, but which are themselves not types,
+  // so they can not be passed here. They should be converted to a type through
+  // an `as type` conversion, such as to a `FacetAccessType` instruction whose
+  // value is of type `TypeType`.
   auto GetTypeIdForTypeInst(SemIR::InstId inst_id) -> SemIR::TypeId {
     return GetTypeIdForTypeConstant(constant_values().Get(inst_id));
   }