[libc++] reduce <complex> parsing time

Instead of including <ios> for ios_base::failbit, simply get failbit
member of the template argument. Print directly to a stream instead
of using intermediate ostringstream.

    Parsing time: 874ms -> 164ms (-81%)

Thanks to Nikita Kniazev for the patch!

Differential Revision: https://reviews.llvm.org/D71214