Add a workaround for a libstdc++-4.2 <tr1/hashtable> bug. This header uses

  return false;

in a function returning a pointer. 'false' was a null pointer constant in C++98
but is not in C++11. Punch a very small hole in the initialization rules in
C++11 mode to allow this specific case in system headers.

llvm-svn: 184395