[libc++] Use correct rand.eng.mers all-zeroes seed sequence fallback

Summary:
When a seed sequence would lead to having no non-zero significant bits
in the initial state of a `mersenne_twister_engine`, the fallback is to
flip the most significant bit of the first value that appears in the
textual representation of the initial state.

rand.eng.mers describes this as setting the value to be 2 to the power
of one less than w; the previous value encoded in the implementation,
namely one less than "2 to the power of w", is replaced by the correct
value in this patch.

Reviewers: mclow.lists, EricWF, jasonliu

Reviewed By: mclow.lists

Subscribers: mclow.lists, jasonliu, EricWF, christof, ldionne, cfe-commits

Differential Revision: https://reviews.llvm.org/D50736

llvm-svn: 339969