GHC claims to lift the H98 restriction that a class method must not
add a constraint on the class type variable (manual 7.4.2.1 "Class
method types").  But the validity check was incorrect in the case
where the class method had a forall.  E.g.
	class C a where
	  op :: forall b. (Show b, Show a) => ...

This commit fixes the bug.

tcrun037, and tcfail149, test with and without -fglasgow-exts.

	MERGE TO STABLE