Make the string_view literal operators constexpr like the ctors they call.

[Jonathan Wakely]

On 8 March 2014 16:29, Ed Smith-Rowland wrote:
> On 03/08/2014 11:27 AM, Ed Smith-Rowland wrote:
>>
>> The title says it all.  This was just an oversight in the original patch.
>>
>> This could wait until stage 1 obviously.  I just wanted to post it.
>>
>> There are a lot of post-Issaquah constexpr changes but these will have to
>> wait for front-end support for C++14 constexpr.
>>
>> Builds and tests clean on x86_64-linux.
>>
>> OK?

OK for stage 1, but not now.  (And please remember to CC gcc-patches)