[IR] Optimize adding attribute to AttributeList (NFC)

When adding an enum attribute to an AttributeList, avoid going
through an AttrBuilder and instead directly add the attribute to
the correct set. Going through AttrBuilder is expensive, because
it requires all string attributes to be reconstructed.

This can be further improved by inserting the attribute at the
right position and using the AttributeSetNode::getSorted() API.

This recovers the small compile-time regression from D94633.