This adds constexpr to 11 algorithms defined in <numeric> as per P1645R1.

libstdc++-v3/ChangeLog:

	P1645R1 constexpr for <numeric> algorithms
	* include/bits/stl_numeric.h (iota, accumulate, inner_product,
	partial_sum, adjacent_difference): Make conditionally constexpr for
	C++20.
	* include/std/numeric (__cpp_lib_constexpr_numeric): Define this feature
	test macro.
	(reduce, transform_reduce, exclusive_scan, inclusive_scan,
	transform_exclusive_scan, transform_inclusive_scan): Make conditionally
	constexpr for C++20.
	* include/std/version (__cpp_lib_constexpr_numeric): Define.
	* testsuite/26_numerics/accumulate/constexpr.cc: New test.
	* testsuite/26_numerics/adjacent_difference/constexpr.cc: Likewise.
	* testsuite/26_numerics/exclusive_scan/constexpr.cc: Likewise.
	* testsuite/26_numerics/inclusive_scan/constexpr.cc: Likewise.
	* testsuite/26_numerics/inner_product/constexpr.cc: Likewise.
	* testsuite/26_numerics/iota/constexpr.cc: Likewise.
	* testsuite/26_numerics/partial_sum/constexpr.cc: Likewise.
	* testsuite/26_numerics/reduce/constexpr.cc: Likewise.
	* testsuite/26_numerics/transform_exclusive_scan/constexpr.cc: Likewise.
	* testsuite/26_numerics/transform_inclusive_scan/constexpr.cc: Likewise.
	* testsuite/26_numerics/transform_reduce/constexpr.cc: Likewise.