[libc++] Error if someone tries to use MSVC and tell them to contact the libc++ developers

Nobody knows if there are users of libc++ with MSVC. Let's try to find that out and encourage them to upstream their changes to make that configuration work.

Reviewed By: ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D129055