Fix SF bug #1167751, Argument genexp corner case

Incorrect code was generated for:

  foo(a = i for i in range(10))

This should have generated a SyntaxError.  Fix the Grammar so
it raises a SyntaxError and test it.

I'm uncertain whether this should be backported.  It makes
something that was Syntactically valid invalid.  However,
the code would either be completely broken or do the wrong thing.