Fix typecheck of default associated type decls
This bug was thrown up by Trac #11361, but I found that the problem was deeper: GHC was allowing class C a where type F (a :: k) :: * type F (x :: *) = x -- Not right! (Which is now indexed-types/should_compile/T11361a.) Anyway the fix is relatively simple; use tcMatchTys in tcDefaultAssocDecl. Merge to 8.0 branch.