Implement partial ordering of class template partial specializations

Implement partial ordering of class template partial specializations
and function templates that contain variadic templates. This involves
three small-ish changes:

  (1) When transforming a pack expansion, if the transformed argument
  still contains unexpanded parameter packs, build a pack
  expansion. This can happen during the substitution that occurs into
  class template partial specialiation template arguments during
  partial ordering.

  (2) When performing template argument deduction where the argument
  is a pack expansion, match against the pattern of that pack
  expansion.

  (3) When performing template argument deduction against a non-pack
  parameter, or a non-expansion template argument, deduction fails if
  the argument itself is a pack expansion (C++0x
  [temp.deduct.type]p22).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@123279 91177308-0d34-0410-b5e6-96231b3b80d8