Reimplement the comparison of a class template partial

Reimplement the comparison of a class template partial
specialization's template arguments against the primary template's
template arguments using the obvious, correct method of checking the
injected-class-name type (C++ [temp.class.spec]p9b3). The previous
incarnation of this comparison attempted to use its own formulation of
the injected-class-name, which is redudant and, with the introduction
of variadic templates, became wrong (again).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@122508 91177308-0d34-0410-b5e6-96231b3b80d8