[PATCH] [libc++] Fix std::make_heap's worst case time complexity

[David Majnemer]

std::make_heap is currently implemented by iteratively applying a
sift_up-type algorithm.
Since sift-up is O(ln n), this gives std::make_heap a worst case time
complexity of O(n ln n).  Since the C++ standard mandates that
std::make_heap make no more than O(3n) comparisons, this makes our
std::make_heap out of spec.

Fix this by introducing an implementation of __sift_down and switch
std::make_heap to create the heap using it.  This gives std::make_heap
linear time complexity in the worst case.

This fixes PR20161.
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