PR libstdc++/56383
	* testsuite/20_util/enable_shared_from_this/56383.cc: New.
	* include/bits/shared_ptr_base.h (__enable_shared_from_this): Make
	friend declaration match previous declaration of
	__enable_shared_from_this_helper.
	* include/bits/shared_ptr.h (enable_shared_from_this): Likewise.

From-SVN: r231181