PR libstdc++/81338 correctly manage string capacity

PR libstdc++/81338 correctly manage string capacity

	PR libstdc++/81338
	* include/bits/basic_string.h [_GLIBCXX_USE_CXX11_ABI] (basic_string):
	Declare basic_stringbuf to be a friend.
	* include/bits/sstream.tcc (basic_stringbuf::overflow)
	[_GLIBCXX_USE_CXX11_ABI]: Use unused capacity before reallocating.
	* include/std/sstream (basic_stringbuf::__xfer_bufptrs): Update string
	length to buffer length.
	* testsuite/27_io/basic_stringstream/assign/81338.cc: New.

From-SVN: r250100