fix PR6811 by not parsing 'super' as a magic expression in

LookupInObjCMethod.  Doing so allows all sorts of invalid code
to slip through to codegen.  This patch does not change the 
AST representation of super, though that would now be a natural
thing to do since it can only be in the receiver position and
in the base of a ObjCPropertyRefExpr.

There are still several ugly areas handling super in the parser,
but this is definitely a step in the right direction.

llvm-svn: 100959