Add noexcept and constexpr to tr2/dynamic_bitset.

[Jonathan Wakely]

On 24 June 2012 12:18, Jonathan Wakely wrote:
> On 24 June 2012 05:00, Ed Smith-Rowland wrote:
>> Subject says it.
>>
>
> This looks good, please CC gcc-patches with a changelog entry and
> confirmation it was tested and it can go in. Thanks.

And don't forget to update the copyright year.