The previous implementation tried to be "efficient" by
storing field names once in IfaceConDecls, and only just
enough information for us to reconstruct the FieldLabel.
But this came at a bit of code complexity cost.
This patch undos the optimization, instead storing a full
FieldLabel at each data constructor. Consequently, this fixes
bugs Trac #12699 and Trac #13250.
Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>