Record full FieldLabel in ifConFields.
Summary: 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 #12699 and #13250. Signed-off-by: Edward Z. Yang <email@example.com> Test Plan: validate Reviewers: adamgundry, bgamari, austin Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D3174