Implement C++20 p1424 - 'constexpr' feature macro concerns.

[Jonathan Wakely]

On 21/08/19 23:36 -0400, Ed Smith-Rowland via libstdc++ wrote:
>On 8/20/19 6:14 PM, Jonathan Wakely wrote:
>>On 16/08/19 22:39 -0400, Ed Smith-Rowland via libstdc++ wrote:
>>>The latest draft and I guess the above paper changed the macro 
>>>names for the C++20 constexpr lib featues.
>>>
>>>__cpp_lib_constexpr_algorithms ->__cpp_lib_constexpr.
>>
>>The __cpp_lib_constexpr macro is (now?) for a different feature.
>>
>>The proposed resolution for https://cplusplus.github.io/LWG/issue3256
>>is to keep the __cpp_lib_constexpr_algorithms name to indicate support
>>for what is currently described by __cpp_lib_constexpr_swap_algorithms
>>(and see https://cplusplus.github.io/LWG/issue3257 for another issue
>>in this area)
>
>I'm testing the attached.
>
>I changed back to __cpp_lib_constexpr_algorithms from __cpp_lib_constexpr.
>
>I dropped__cpp_lib_constexpr_swap_algorithms in favor of 
>__cpp_lib_constexpr_algorithms.
>
>I'm leaving __cpp_lib_constexpr alone.?? We don't test this anyway.?? 
>It's sitting in <version>.
>
>Is it possible that __cpp_lib_constexpr_char_traits also should be 
>bumped from 201611 to 201806 because of P1032 section 10? Note that 
>I'm not done with this but I'd like to get ahead of the macro 
>issues.?? Presumably, __cpp_lib_constexpr_algorithms gets bumped.

Looks good, thanks.

OK for trunk.