PR libstdc++/85818 ensure path::preferred_separator is defined

PR libstdc++/85818 ensure path::preferred_separator is defined

Because path.cc is compiled with -std=gnu++17 the static constexpr
data member is implicitly 'inline' and so no definition gets emitted
unless it gets used in that translation unit. Other translation units
built as C++11 or C++14 still require a namespace-scope definition of
the variable, so mark the definition as used.

	PR libstdc++/85818
	* src/filesystem/path.cc (path::preferred_separator): Add used
	attribute.
	* testsuite/experimental/filesystem/path/preferred_separator.cc: New.

From-SVN: r260326