[libc++] Implement LWG 3199

Summary:
The resolution of LWG 3199 makes sure that input-streaming into an empty bitset
does not set the failbit on the input stream.

Reviewers: mclow.lists, EricWF

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D65105

llvm-svn: 369422