Enable building Cabal with parsec

Cabal's parser has been rewritten in terms of Parsec (which is not
enabled yet in Cabal-2.0 by default, but can be enabled by a cabal
flag). The plan for Cabal is to drop support for the non-parsec parser,
so we need to prepare GHC to cope with new situation.

However, this means that lib:Cabal requires three new library
dependency submodules,

 - parsec
 - text
 - mtl

What complicates matters is that we need to build `ghc-cabal` early on
during the bootstrap phase which currently needs to invoke `ghc --make`
directly. So these additional dependencies need to be integrated into
the monolithic `ghc --make` invocation which produces the `ghc-cabal`
executable.

Test Plan: `./validate --fast` passed

Reviewers: austin, bgamari

Subscribers: erikd, phadej, rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3757

1 files changed, 0 insertions, 0 deletions

diff --git a/libraries/mtl b/libraries/mtl
new file mode 160000
+Subproject b4725fe28cba8a535e969e0ddbce3d5e05146cc