PR78702 fix accessibility of locale::facet::__shim

	PR libstdc++/78702
	* include/bits/locale_classes.h (locale::facet::__shim): Change from
	private to protected.
	* src/c++11/cxx11-shim_facets.cc (__shim_accessor): Define helper to
	make locale::facet::__shim accessible.

From-SVN: r244491