Expose enabled language extensions to TH

This exposes `template-haskell` functions for querying the language
extensions which are enabled when compiling a module,

- an `isExtEnabled` function to check whether an extension is enabled
- an `extsEnabled` function to obtain a full list of enabled extensions

To avoid code duplication this adds a `GHC.LanguageExtensions` module to
`ghc-boot` and moves `DynFlags.ExtensionFlag` into it. A happy
consequence of this is that the ungainly `DynFlags` lost around 500
lines. Moreover, flags corresponding to language extensions are now
clearly distinguished from other flags due to the `LangExt.*` prefix.

Updates haddock submodule.

This fixes #10820.

Test Plan: validate

Reviewers: austin, spinda, hvr, goldfire, alanz

Reviewed By: goldfire

Subscribers: mpickering, RyanGlScott, hvr, simonpj, thomie

Differential Revision: https://phabricator.haskell.org/D1200

GHC Trac Issues: #10820

1 files changed, 2 insertions, 1 deletions

diff --git a/compiler/main/GhcMake.hs b/compiler/main/GhcMake.hs
index d390c16817..f0dc3005f1 100644
--- a/compiler/main/GhcMake.hs
+++ b/compiler/main/GhcMake.hs
@@ -54,6 +54,7 @@ import StringBuffer
 import SysTools
 import UniqFM
 import Util
+import qualified GHC.LanguageExtensions as LangExt
 
 import Data.Either ( rights, partitionEithers )
 import qualified Data.Map as Map
@@ -1986,7 +1987,7 @@ preprocessFile hsc_env src_fn mb_phase (Just (buf, _time))
                 | Just (Unlit _) <- mb_phase    = True
                 | Nothing <- mb_phase, Unlit _ <- startPhase src_fn  = True
                   -- note: local_opts is only required if there's no Unlit phase
-                | xopt Opt_Cpp dflags'          = True
+                | xopt LangExt.Cpp dflags'      = True
                 | gopt Opt_Pp  dflags'          = True
                 | otherwise                     = False