Template argument deduction for template template parameters. This

permits, among other things, ripping apart and reconstructing
templates via partial specialization:

  template<typename T> 
  struct DeepRemoveConst { typedef T type; };

  template<typename T>
  struct DeepRemoveConst<const T> {
    typedef typename DeepRemoveConst<T>::type type;
  };

  template<template<typename> class TT, typename T>
  struct DeepRemoveConst<TT<T> > {
    typedef TT<typename DeepRemoveConst<T>::type> type;
  };

Also, fix a longstanding thinko in the code handling partial ordering
of class template partial specializations. We were performing the
second deduction without clearing out the results of the first
deduction. It's amazing we got through so much code with such a
horrendous error :(

llvm-svn: 86893