libstdc++: Tweaks for std::format fast path

libstdc++: Tweaks for std::format fast path

Fix an incorrect call to _Sink::_M_reserve() which should have passed
the __n parameter. This was not actually a problem because it was in an
discarded statement, because only the _Seq_sink<basic_string<C>>
specialization was used.

Also add some branch prediction hints, explanatory comments, and debug
mode assertions to _Seq_sink.

libstdc++-v3/ChangeLog:

	* include/std/format (_Seq_sink): Fix missing argument in
	discarded statement. Add comments, likely/unlikely attributes
	and debug assertions as sanity checks.