libc++: Race condition in facets initialization?

I am in a very similar position, having done very similar work to get libc++ working under MSVC so I appreciate the difficulty of providing a reproduction.
However I think Howard is almost certainly right and a lack of constexpr is at the root of the problem.
MSVC runtime does not attempt to initialize stdin stdout and therefore cin cout until they are used in a Windows application. One thing to try may be changing your build to target the Console. No promises but it does change the related static init.