Include a stack trace in the panic message, when GHC is compiled profiled.

I tried this out on the panic we're currently getting for #3103:

ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 7.3.20111128 for x86_64-unknown-linux):
        tcIfaceGlobal (local): not found:
    base:GHC.Word.W#{d 6w}
    [(32R, Type constructor `base:GHC.Word.Word{tc 32R}'),
     (r6O, Identifier `base:GHC.Word.$fNumWord{v r6O}'),
     (r6P, Identifier `base:GHC.Word.$fEqWord{v r6P}'),
     (r6Q, Identifier `base:GHC.Word.$fNumWord1{v r6Q}'),
     (r6R, Identifier `base:GHC.Word.$fNumWord2{v r6R}'),
     (r6S, Data constructor `base:GHC.Word.W#{d r6S}'),
     (r6U, Identifier `base:GHC.Word.W#{v r6U}'),
     (r75, Identifier `base:GHC.Word.$fNumWord_$csignum{v r75}'),
     (r76, Identifier `base:GHC.Word.$fEqWord_$c/={v r76}'),
     (r77, Identifier `base:GHC.Word.$fEqWord_$c=={v r77}')]
{ Main.main
   GHC.defaultErrorHandler
    GHC.runGhc
     GhcMonad.>>=
      GhcMonad.>>=.\
       Main.main'
        Main.doMake
         GhcMake.load
          GhcMake.load2
           GhcMake.upsweep
            GhcMake.upsweep.upsweep'
             GhcMake.reTypecheckLoop
              GhcMake.typecheckLoop
               GhcMake.typecheckLoop.\
                TcRnMonad.initIfaceCheck
                 TcRnMonad.initTcRnIf
                  IOEnv.runIOEnv
                   IOEnv.thenM
                    IOEnv.thenM.\
                     TcIface.typecheckIface
                      TcIface.typecheckIface.\
                       LoadIface.loadDecls
                        LoadIface.loadDecl
                         TcIface.tcIfaceDecl
                          TcIface.tc_iface_decl
                           TcIface.tcIdInfo
                            MonadUtils.foldlM
                             TcIface.tcIdInfo.tcPrag
                              TcIface.tcUnfolding
                               TcIface.tcPragExpr
                                TcIface.tcIfaceExpr
                                 TcIface.tcIfaceAlt
                                  TcIface.tcIfaceDataCon }

compiler/utils/Panic.lhs

@@ -35,7 +35,7 @@ import Control.Concurrent ( MVar, ThreadId, withMVar, newMVar, modifyMVar_,
                             myThreadId )
 import Data.Dynamic
 import Debug.Trace	  ( trace )
-import System.IO.Unsafe	  ( unsafePerformIO )
+import System.IO.Unsafe
 import System.Exit
 import System.Environment
 
@@ -47,6 +47,9 @@ import System.Posix.Signals
 import GHC.ConsoleHandler
 #endif
 
+#if __GLASGOW_HASKELL__ >= 703
+import GHC.Stack
+#endif
 
 -- | GHC's own exception type 
 --   error messages all take the form:
@@ -160,7 +163,16 @@ handleGhcException = ghandle
 
 -- | Panics and asserts.
 panic, sorry, pgmError :: String -> a
+#if __GLASGOW_HASKELL__ >= 703
+panic    x = unsafeDupablePerformIO $ do
+   stack <- ccsToStrings =<< getCurrentCCS x
+   if null stack
+      then throwGhcException (Panic x)
+      else throwGhcException (Panic (x ++ '\n' : renderStack stack))
+#else
 panic    x = throwGhcException (Panic x)
+#endif
+
 sorry    x = throwGhcException (Sorry x)
 pgmError x = throwGhcException (ProgramError x)