Generate real (but empty) object files for signatures.

Summary:
It's not great, but it preserves a nice invariant that every Haskell
source file has an object file (we already have a hack in place ensure
this is the case for hs-boot files) and further ensures every package
has a library associated with it (which would not be the case if
the package had all signatures and we didn't make object files.)

Contains Cabal submodule update.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: simonpj, austin

Subscribers: carter, thomie

Differential Revision: https://phabricator.haskell.org/D548

1 files changed, 0 insertions, 0 deletions

diff --git a/libraries/Cabal b/libraries/Cabal
-Subproject 6c395bb8f22961ce5267df64e6d9351c310fcbb
+Subproject ea062bf522e015f6e643bcc833487098edba839