[C++11 Compat] Fix breaking change in C++11 pair copyctor.

While this code is valid C++98, it is not valid C++11. The problem can be
reduced to:

class MDNode;

class DIType {
  operator MDNode*() const {return 0;}
};

class WeakVH {
  WeakVH(MDNode*) {}
};

int main() {
  DIType di;
  std::pair<void*, WeakVH> p(std::make_pair((void*)0, di)));
}

This was not detected by any of the bots we have because they either compile
C++98 with libstdc++ (which allows it), or C++11 with libc++ (which incorrectly
allows it). I ran into the problem when compiling with VS 2012 RC.

Thanks to Richard for explaining the issue.

llvm-svn: 158245