Explicit specialization needed for cpp14_promoted<long double, long double> #11

Hi,

The partial specializations provided for cpp14_promoted in cx_math.h (https://github.com/elbeno/constexpr/blob/master/src/include/cx_math.h#L872) are ambiguous.

This can also be seen by testing with some main stream compilers: https://godbolt.org/z/rGfcGzKfh

The easiest fix is to provide an explicit specialization for when the types are both long double:

  template <  >
  struct cpp14_promoted < long double , long double >
  {
    using type = long double ;
  } ;