Use assembler-with-cpp mode when running CPP.

This is needed because Clang is very strict about C99 macro rules, which
dictate that '#' in a body must have a token immediately following it for
string-ification. In practice we break this all the time, because we do
very weird stuff like:

    #define FOOBAR(xyz) \
    {-# SOME PRAGMA #-} \
    baz :: (xyz) \
    baz = ...

where the leading '#' in in the macro body clearly breaks this rule.

Signed-off-by: Austin Seipp <aseipp@pobox.com>

compiler/main/DriverPipeline.hs

@@ -1140,10 +1140,10 @@ runPhase (RealPhase cc_phase) input_fn dflags
                 -- very weakly typed, being derived from C--.
                 ["-fno-strict-aliasing"]
 
-        let gcc_lang_opt | cc_phase `eqPhase` Ccpp  = "c++"
-                         | cc_phase `eqPhase` Cobjc = "objective-c"
+        let gcc_lang_opt | cc_phase `eqPhase` Ccpp    = "c++"
+                         | cc_phase `eqPhase` Cobjc   = "objective-c"
                          | cc_phase `eqPhase` Cobjcpp = "objective-c++"
-                         | otherwise                = "c"
+                         | otherwise                  = "c"
         liftIO $ SysTools.runCc dflags (
                 -- force the C compiler to interpret this file as C when
                 -- compiling .hc files, by adding the -x c option.
@@ -2042,8 +2042,12 @@ doCpp dflags raw input_fn output_fn = do
                     ++ map SysTools.Option backend_defs
                     ++ map SysTools.Option hscpp_opts
                     ++ map SysTools.Option sse_defs
+        -- Set the language mode to assembler-with-cpp when preprocessing. This
+        -- alleviates some of the C99 macro rules relating to whitespace and the hash
+        -- operator, which we tend to abuse. Clang in particular is not very happy
+        -- about this.
                     ++ [ SysTools.Option     "-x"
-                       , SysTools.Option     "c"
+                       , SysTools.Option     "assembler-with-cpp"
                        , SysTools.Option     input_fn
         -- We hackily use Option instead of FileOption here, so that the file
         -- name is not back-slashed on Windows.  cpp is capable of