Adjust std::rotl, std::rotr etc to match final P0553R4 proposal

Adjust std::rotl, std::rotr etc to match final P0553R4 proposal

This proposal has now been accepted for C++20, with a few changes. This
patch adjusts std::rotl and std::rotr to match the final specification
and declares the additions for C++2a mode even when __STRICT_ANSI__ is
defined.

	* include/std/bit (__rotl, __rotr): Change second parameter from
	unsigned int to int and handle negative values.
	(rotl, rotr): Remove check for __STRICT_ANSI__. Change second
	parameter from unsigned int to int. Add nodiscard attribute.
	* testsuite/26_numerics/bit/bitops.rot/rotl.cc: Rename to ...
	* testsuite/26_numerics/bit/bit.rotate/rotl.cc: Here. Test negative
	shifts.
	* testsuite/26_numerics/bit/bitops.rot/rotr.cc: Rename to ...
	* testsuite/26_numerics/bit/bit.rotate/rotr.cc: Here. Test negative
	shifts.

From-SVN: r273706