Implement P0415 More constexpr for std::complex.

Implement P0415 More constexpr for std::complex.

2018-11-23  Edward Smith-Rowland  <3dw4rd@verizon.net>

	Implement P0415 More constexpr for std::complex.
	* include/std/complex (conj(complex<Tp>), norm(complex<Tp>)): Constexpr;
	(real(Tp), imag(Tp)): Constexpr;
	(operator@=(Tp), operator@=(complex<Tp>)): Constexpr;
	(operator@(Tp,complex<Tp>), operator@(complex<Tp>,Tp)
	operator@(complex<Tp>,complex<Tp>)): Constexpr.
	* testsuite/26_numerics/complex/comparison_operators/
	more_constexpr.cc: New test.
	* testsuite/26_numerics/complex/operators/more_constexpr.cc: New test.
	* testsuite/26_numerics/complex/requirements/
	more_constexpr.cc: New test.
	* testsuite/26_numerics/complex/value_operations/
	more_constexpr.cc: New test.
	* testsuite/26_numerics/headers/complex/synopsis.cc:
	Add _GLIBCXX20_CONSTEXPR to applicable operators; Add missing proj().
	* testsuite/26_numerics/headers/complex/synopsis.cc:
	Add _GLIBCXX20_CONSTEXPR to relevant decls.

From-SVN: r266416