The copy assignment operator for recursive_directory_iterator was not
exported despite being needed. The __shared_ptr default constructors are
not needed when compiling with GCC but Clang requires them for -O1.

	PR libstdc++/91067
	* acinclude.m4 (libtool_VERSION): Bump to 6:27:0.
	* configure: Regenerate.
	* config/abi/pre/gnu.ver (GLIBCXX_3.4.27): Add new version. Export
	missing symbols.
	* testsuite/27_io/filesystem/iterators/91067.cc: New test.
	* testsuite/util/testsuite_abi.cc: Add new symbol version.

From-SVN: r273023