libstdc++: Fix error in filesystem::path with Clang

THis fixes teh following error seen with Clang:

error: function '_S_convert<std::basic_string_view<char8_t>>' with deduced
return type cannot be used before it is defined
          return string_type(_S_convert(std::u8string_view(__str)));
                             ^

libstdc++-v3/ChangeLog:

	* include/bits/fs_path.h (path::_S_convert(T)): Avoid recursive
	call to function with deduced return type.