libstdc++: -Wswitch and ios::openmode

In addition to marking it as flag_enum, we want to avoid warnings about
not having a case for the implementation detail enumerators
_S_ios_openmode_*.  And also for _S_noreplace in standard modes before it
was added.

libstdc++-v3/ChangeLog:

	* include/bits/ios_base.h (_GLIBCXX_NOREPLACE_UNUSED): New.
	(_Ios_Openmode): Add unused attributes.
	* testsuite/27_io/ios_base/types/openmode/case_label.cc: Handle
	noreplace.