Introduce TypeInstId (#5288)

TypeInstId is an InstId whose constant value has a type of TypeType.
This includes:
- Type value instructions, the `ClassType` or `IntLiteralType`
instructions.
- Constraint value instructions, which are the `FacetType` and
`TypeType` instructions, each of which also have type TypeType.

TypeInstId encodes in the type system that it is safe to convert the
instruction's value to a TypeId, and CHECKs at construction that this
invariant is maintained.

---------

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

toolchain/check/handle_binding_pattern.cpp

@@ -143,7 +143,7 @@ static auto HandleAnyBindingPattern(Context& context, Parse::NodeId node_id,
                                          cast_type_id);
         });
     if (cast_type_id == SemIR::ErrorInst::SingletonTypeId) {
-      cast_type_inst_id = SemIR::ErrorInst::SingletonInstId;
+      cast_type_inst_id = SemIR::ErrorInst::SingletonTypeInstId;
     }
     auto binding_id =
         context.parse_tree().As<Parse::VarBindingPatternId>(node_id);