Merge #10645: Use nullptr (C++11) instead of zero (0) as the null pointer constant

Merge #10645: Use nullptr (C++11) instead of zero (0) as the null pointer constant

36d326e8b Use nullptr instead of zero (0) as the null pointer constant (practicalswift)

Pull request description:

  Use `nullptr` instead of zero (0) as the null pointer constant.

  The road towards `nullptr` (C++11) is split into two PRs:
  * `NULL` → `nullptr` is handled in PR #10483 (scripted)
  * `0` → `nullptr` is handled in PR #10645 (manual, this PR)

  By using the C++11 keyword `nullptr` we are guaranteed a prvalue of type `std::nullptr_t`.

  For a more thorough discussion, see "A name for the null pointer: nullptr" (Sutter &
  Stroustrup), http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2431.pdf

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