LWG 2996 add rvalue overloads for shared_ptr aliasing and casting

LWG 2996 add rvalue overloads for shared_ptr aliasing and casting

	* doc/xml/manual/intro.xml: Document LWG DR 2996 change.
	* doc/html/*: Regenerate.
	* include/bits/shared_ptr.h (shared_ptr(shared_ptr&&, T*)): Add
	rvalue aliasing constructor.
	(static_pointer_cast, const_pointer, dynamic_pointer_cast)
	(reinterpret_pointer_cast): Add overloads taking rvalues.
	* include/bits/shared_ptr_base.h (__shared_ptr(__shared_ptr&&, T*)):
	Add rvalue aliasing constructor.
	* testsuite/20_util/shared_ptr/casts/1.cc: Change "compile" test to
	"run" and check return values as well as types.
	* testsuite/20_util/shared_ptr/casts/reinterpret.cc: Likewise.
	* testsuite/20_util/shared_ptr/casts/rval.cc: New test.
	* testsuite/20_util/shared_ptr/cons/alias-rval.cc: New test.
	* testsuite/20_util/shared_ptr/cons/alias.cc: Remove unused return
	values.

From-SVN: r271583