In this patch

    commit 6eabb6dd
    Author: Simon Peyton Jones <simonpj@microsoft.com>
    Date:   Tue Dec 15 14:26:13 2015 +0000

    Allow recursive (undecidable) superclasses

I changed (transSuperClasses p) to return only the
superclasses of p, but not p itself. (Previously it always
returned p as well.)

The use of transSuperClasses in TcErrors.warnRedundantConstraints
really needs 'p' in the result -- but I faild to fix this
call site, and instead crippled the test for Trac #10100.

This patch sets things right

* Accomodates the API change
* Re-enables T10100
* And thereby fixes Trac #11474