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

[Jonathan Wakely]

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).