I got a report that Clang fails to compile the filesystem lib, with
the following error:

/home/jwakely/gcc/latest/include/c++/6.0.0/experimental/bits/fs_path.h:563:18: fatal error: explicit specialization of 'std::experimental::filesystem::v1::path::__is_encoded_char<char>' after instantiation
    struct path::__is_encoded_char<char> : std::true_type
                 ^~~~~~~~~~~~~~~~~~~~~~~
/home/jwakely/gcc/latest/include/c++/6.0.0/experimental/bits/fs_path.h:104:9: note: implicit instantiation first required here
      : decltype(__is_path_src(std::declval<_Source>(), 0))
        ^

The problem is that the path::_Cmpt constructors do overload
resolution on the path constructors, which requires the explicit
specializations of __is_encoded_char. Solved by moving the definition
of _Cmpt after the specializations.

I'm still reducing it to report a g++ bug for the missed diagnostic.

Tested powerpc64le-linux, committed to trunk. This is unfortunately
too late for 5.3, so I'll fix it on the branch when it reopens.