PR libstdc++/91067
	* acinclude.m4 (libtool_VERSION): Bump to 6:28:0.
	* configure: Regenerate.
	* config/abi/pre/gnu.ver (GLIBCXX_3.4.28): Add new version. Export
	missing symbols.
	* testsuite/27_io/filesystem/iterators/91067.cc: Test move
	constructors.
	* testsuite/util/testsuite_abi.cc: Add new symbol version.

From-SVN: r275032