PR libstdc++/53984
	* include/bits/basic_ios.h (basic_ios::_M_setstate): Adjust comment.
	* include/bits/istream.tcc (basic_istream::sentry): Handle exceptions
	during construction.
	* include/std/istream: Adjust comments for formatted input functions
	and unformatted input functions.
	* testsuite/27_io/basic_fstream/53984.cc: New.
	* testsuite/27_io/basic_istream/sentry/char/53984.cc: New.

From-SVN: r250545