[libc++] "Bottom-up heapsort" improvement to sort_heap.

https://en.wikipedia.org/wiki/Heapsort#Bottom-up_heapsort
In `pop_heap` specifically, the item we insert at the top and
sift downward is guaranteed to be leaf-sized, so we expect it
to go pretty far down. Sift it down as if it were INT_MIN, and
then bubble it back up if needed.
Also known as "heapsort with bounce."

Numbers are here: https://godbolt.org/z/cvfnYW6fe

Fixes #10008.

Differential Revision: https://reviews.llvm.org/D118003