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Suggested by Claus Reinke

pointed out by Claus Reinke

Allows us to use a different name base for foo.stderr for old compilers, e.g.
test('tc141',
     namebase_if_compiler_lt('ghc', '6.9', 'tc141-6.8'),
     compile_fail,
     [''])

* Code of #1776 from aslatter@gmail.com

I'm actually not certain why this output has changed:
  
-      Code: case reverse "no" of [] -> return (GHC.Base.[])
+      Code: let
+	      return = return
+	      $dMonad = Language.Haskell.TH.Syntax.$f22
+	    in case reverse "no" of [] -> return (GHC.Base.[])

  Perhaps it's something to do with inlining?
  
  Anyway it's not too important, because this stuff is only displayed
  for TH splices that crash with (head []) or something like that.

* The error message has become less informative, but I don't see how to avoid
  this easily.
* The error message is now also different to that of 6.8.  I am not sure how
  to handle this case.  Can we have different stderr files for different
  compiler versions?  Or shall we simply merge the corresponding changes
  into 6.8?

* Demonstrates a bug in TcUnify.tcSubFun found by Andrew Appleyard

The rationale for these new behaviors is as follows:

We now allow existentials to be unpacked in local (but not top-level!)
ValBinds, so some pattern signatures that used to fail in the renamer
now fail in the type checker (if the type of the body mentions the
unpacked type var):
   tc141(normal)

We don't always print HsWrappers in debug mode:
   TH_exn1(normal)
   TH_exn2(normal)

Sections are parsed (and therefore printed) differently:
   rnfail019(normal)
   read023(normal)

Character numbers in pattern printing are more precise:
   tcfail067(normal)
   tcfail124(normal)
   tcfail125(normal)