AST: Stricter contract for Type::subst()

Fix an odd corner case when UseErrorTypes was off; we would
return the empty type if dependent member type substitution
failed, but otherwise return the original type if it was a
generic type parameter or an archetype.

Now, if UseErrorTypes is off, return the empty type in both
cases, even if the original type is 'primary'.

Author: slavapestov

lib/AST/Type.cpp

@@ -2971,7 +2971,7 @@ static Type substType(Type derivedType,
     if (isa<GenericTypeParamType>(substOrig)) {
       if (options.contains(SubstFlags::UseErrorType))
         return ErrorType::get(type);
-      return Type(type);
+      return Type();
     }
 
     auto archetype = cast<ArchetypeType>(substOrig);
@@ -2987,7 +2987,7 @@ static Type substType(Type derivedType,
     if (!parent) {
       if (options.contains(SubstFlags::UseErrorType))
         return ErrorType::get(type);
-      return Type(type);
+      return Type();
     }
 
     // Substitute into the parent type.