[PATCH, libstdc++] Implement P0415 More constexpr for std::complex.

[Ed Smith-Rowland <3dw4rd at verizon dot net>]

Greetings,

This is late but I wanted to put it out there just to finish a thing.

It's fairly straightforward constexpr of operators and some simple 
functions for std::complex.

The only thing that jumped out was the norm function.  We had this:

    struct _Norm_helper<true>
    {
      template<typename _Tp>
        static inline _Tp _S_do_it(const complex<_Tp>& __z)
        {
          _Tp __res = std::abs(__z);
          return __res * __res;
        }
    };

Since abs can't be made constexpr for complex since it involves sqrt (It 
probably could but that's another story) I had to fall back to the x^2 + 
y^2.  I don't know who that will bother.  This version should be faster 
and I can't think of any useful trustworthy difference numerically 
either in terms of accuracy of stability.

Barring any feedback on that I'll clean it up and maybe rename my tests 
from constexpr_all_the_things.cc to more_constexpr.cc ;-)

It builds and tests cleanly on x86_64-linux.

Ed

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