libc++ commits to be merged to the 3.8 release branch

[Hans Wennborg via cfe-commits]

Thanks! Merged as indicated below.

On Wed, Jan 13, 2016 at 3:48 PM, Marshall Clow <mclow.lists at gmail.com> wrote:
> r257702 | marshall | 2016-01-13 15:05:15 -0800 (Wed, 13 Jan 2016) | 1 line
>
> Better comments in test. NFC

r257721.

> r257696 | marshall | 2016-01-13 14:52:36 -0800 (Wed, 13 Jan 2016) | 1 line
>
> Fix test for C++03 - lacking noexcept

r257720.

> r257682 | marshall | 2016-01-13 13:54:34 -0800 (Wed, 13 Jan 2016) | 1 line
>
> Fix PR#25973 : 'basic_string::assign(InputIt, InputIt) doesn't provide the
> strong exception safety guarantee'. This turned out to be a pervasive
> problem in <string>, which required a fair amount of rework. Add in an
> optimization for when iterators provide noexcept
> increment/comparison/assignment/dereference (which covers many of the
> iterators in libc++). Reviewed as http://reviews.llvm.org/D15862

r257716.