The current generic implementation of __complex_proj used when cproj is
not available calculates the wrong projection, giving a different result
than given by C99's cproj.

When C99 cproj is not available but isinf and copysign are, use those to
give correct results for float, double and long double. Otherwise, and
for other specializations of std::complex, just use a generic version
that returns its argument, and so doesn't support infinities.

We might want to consider adding additional overloads of __complex_proj
to support extended types such as _Float64x, _Float128 etc.

	PR libstdc++/61761
	* include/std/complex (__complex_proj): Return parameter unchanged.
	[_GLIBCXX_USE_C99_COMPLEX] (__complex_proj): Change overloads for
	floating-point types to take std::complex arguments.
	[_GLIBCXX_USE_C99_MATH_TR1] (__complex_proj): Add overloads for
	floating-point types.
	* testsuite/26_numerics/complex/proj.cc: New test.

From-SVN: r270759