libstdc++: Change return type of std::bit_width to int (LWG 3656)

libstdc++-v3/ChangeLog:

	* doc/html/manual/bugs.html: Regenerate.
	* doc/xml/manual/intro.xml: Document LWG 3656 change.
	* include/std/bit (__bit_width, bit_width): Return int.
	* testsuite/26_numerics/bit/bit.pow.two/lwg3656.cc: New test.
4 files changed