Merge "Remove invalid constexpr until we get to C++17."

constexpr void (*xmlDeleter)(T* t);

template <>

constexpr auto xmlDeleter<xmlDoc> = xmlFreeDoc;

// http://b/111067277 - Add back constexpr when we switch to C++17.

template <>

constexpr auto xmlDeleter<xmlChar> = [](xmlChar *s) { xmlFree(s); };

auto xmlDeleter<xmlChar> = [](xmlChar *s) { xmlFree(s); };

/** @return a unique_ptr with the correct deleter for the libxml2 object. */

template <class T>