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/import_cpp.cpp

@@ -272,7 +272,7 @@ static auto MapType(Context& context, clang::QualType type) -> TypeExpr {
       context.ast_context().getTypeSize(type) == 32) {
     return MakeIntType(context, context.ints().Add(32));
   }
-  return {.inst_id = SemIR::ErrorInst::SingletonInstId,
+  return {.inst_id = SemIR::ErrorInst::SingletonTypeInstId,
           .type_id = SemIR::ErrorInst::SingletonTypeId};
 }