[libc++] Fix Windows build error in include/filesystem

_LIBCPP_FUNC_VIS is redundant since the class is already annotated with
_LIBCPP_EXCEPTION_ABI.

Fixes this build error:

    In file included from fstream:188:
    filesystem(1350,3):  error: attribute 'dllimport' cannot be applied to member of 'dllimport' class
      _LIBCPP_FUNC_VIS
    __config(674,37):  note: expanded from macro '_LIBCPP_FUNC_VIS'
    #define _LIBCPP_FUNC_VIS            _LIBCPP_DLL_VIS
    __config(666,38):  note: expanded from macro '_LIBCPP_DLL_VIS'
    #  define _LIBCPP_DLL_VIS __declspec(dllimport)
    filesystem(1313,7):  note: previous attribute is here
    class _LIBCPP_EXCEPTION_ABI filesystem_error : public system_error {
    __config(675,37):  note: expanded from macro '_LIBCPP_EXCEPTION_ABI'
    #define _LIBCPP_EXCEPTION_ABI       _LIBCPP_DLL_VIS
    __config(666,38):  note: expanded from macro '_LIBCPP_DLL_VIS'
    #  define _LIBCPP_DLL_VIS __declspec(dllimport)

Differential Revision: https://reviews.llvm.org/D57354

llvm-svn: 352525