Introduce an egregious hack to work around a bug in libstdc++ 4.2.x's

<tr1/hashtable> header, where a friend class template
std::tr1::__detail::_Map_base is declared with the wrong template
parameters. GCC doesn't catch the problem, so Clang does a little
back-flip to avoid diagnosing just this one instance of the problem.

llvm-svn: 100790