libstdc++: Fix exception handling in std::ostream seek functions

libstdc++: Fix exception handling in std::ostream seek functions

N3168 added the requirement that the [ostream.seeks] functions create a
sentry object. Nothing in the requirements of those functions says
anything about catching exceptions and setting badbit.

As well as not catching exceptions, this change results in another
observable behaviour change. Previously seeking on a stream with eofbit
set would work (as long as badbit and failbit weren't set). The
construction of a sentry causes failbit to be set when eofbit is set,
which causes the seek to fail. It is necessary to clear the eofbit
before seeking now.

libstdc++-v3/ChangeLog:

	* include/bits/ostream.tcc (sentry): Only set failbit if badbit
	is set, not if eofbit is set.
	(tellp, seekp, seekp): Create sentry object. Do not set badbit
	on exceptions.
	* testsuite/27_io/basic_ostream/seekp/char/exceptions_badbit_throw.cc:
	Adjust expected behaviour.
	* testsuite/27_io/basic_ostream/seekp/wchar_t/exceptions_badbit_throw.cc:
	Likewise.
	* testsuite/27_io/basic_ostream/tellp/char/exceptions_badbit_throw.cc:
	Likewise.
	* testsuite/27_io/basic_ostream/tellp/wchar_t/exceptions_badbit_throw.cc:
	Likewise.
	* testsuite/27_io/basic_ostream/seekp/char/n3168.cc: New test.
	* testsuite/27_io/basic_ostream/seekp/wchar_t/n3168.cc: New test.
	* testsuite/27_io/basic_ostream/tellp/char/n3168.cc: New test.
	* testsuite/27_io/basic_ostream/tellp/wchar_t/n3168.cc: New test.