This patch is more than just adding the `constexpr` keyword, because
the old code relied on `goto`, and `goto` is not constexpr-friendly.
Refactor to eliminate `goto`, and then mark it as constexpr in C++20.
I freely admit that the name `__nth_element_partloop` is bad;
I couldn't find any better name because I don't really know
what this loop is doing, conceptually. Vice versa, I think
`__nth_element_find_guard` has a decent name.
Now the only one we're still missing from P0879 is `sort`.
Differential Revision: https://reviews.llvm.org/D93557
207d4be4d9