PR libstdc++/70607 make proj(T) and conj(T) return complex<T>

PR libstdc++/70607 make proj(T) and conj(T) return complex<T>

	PR libstdc++/61791
	PR libstdc++/70607
	* include/std/complex (real(T), imag(T)): Add _GLIBCXX_CONSTEXPR.
	(proj(T), conj(T)): Change return types per DR 1522.
	* include/tr1/complex (conj): Remove overloads and use std::conj.
	* testsuite/26_numerics/complex/dr781_dr1137.cc: Rename to...
	* testsuite/26_numerics/complex/dr781.cc: ... this, and update.
	* testsuite/26_numerics/complex/value_operations/constexpr2.cc: Test
	real(T) and imag(T). Allow testing for C++11 too.

From-SVN: r244900