PR libstdc++/56166
	PR libstdc++/77582
	* include/bits/basic_string.h (basic_string::clear()): Drop reference
	and use empty rep.
	* include/ext/rc_string_base.h (__rc_string_base::_M_clear()):
	Likewise.
	* testsuite/21_strings/basic_string/56166.cc: New.
	* testsuite/ext/vstring/modifiers/clear/56166.cc: New.

From-SVN: r240447