libstdc++: Use C++20 constraints in <bit>

libstdc++: Use C++20 constraints in <bit>

Using a type-constraint instead of enable_if_t in the return types of
these functions greatly reduces the size of their mangled names.  And by
now it's probably safe to assume recent compilers have sufficient C++20
concepts support to handle such constraints.

libstdc++-v3/ChangeLog:

	* include/std/bit: Include <concepts>.
	(byteswap): Use a type-constraint instead of enable_if_t
	inside the return type, and use std::integral.
	(_If_is_unsigned_integer): Replace with ...
	(__unsigned_integer): ... this.
	(rotl): Use a type-constraint instead of enable_if_t inside
	the return type.
	(countl_zero): Likewise.
	(countl_one): Likewise.
	(countr_zero): Likewise.
	(countr_one): Likewise.
	(popcount): Likewise.
	(has_single_bit): Likewise.
	(bit_ceil): Likewise.
	(bit_floor): Likewise.
	(bit_width): Likewise.