Prototype implementation of P1004R2 (making std::vector constexpr).

[ldionne]

Summary

This is the prototype implementation that I was using to test the compiler-side
support for P0784R7. I'm hoping someone else will either pick this up and
finish it, or use it as inspiration for a real implementation of the library
side of P1004R2 in libc++.

Depends on D68364.

[rsmith]

Move some changes from here to the std::allocator change.

[ldionne]

I'll pick this up. I wasn't aware you had started working on this, but thanks!

[ldionne]

I've had a revision locally for a long long time. It doesn't pass everything
yet, but I've managed to get it down to ~10 failing tests with a few more hacks
locally.

[miscco]

Some quick review of the the acutal changes to the headers

libcxx/include/memory

1390–1401

I am wondering whether we should reuse the simplementation of std::copy_n here (or at the call site for what its worth)

1748

As said above, we should be able to reuse the internals of std::copy_n here

1984

From here and below there is _LIBCPP_CONSTEXPR_AFTER_CXX11 rather than _LIBCPP_CONSTEXPR_AFTER_CXX17

Also the ordering of _LIBCPP_INLINE_VISIBILITY and _LIBCPP_CONSTEXPR_AFTER_CXX11 is swapped.

2102

I believe the inline is superfluous with the _LIBCPP_CONSTEXPR_AFTER_CXX17

Appears throughout

libcxx/include/vector

422

Many inline below here