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

[thomasanderson]

Summary

_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)

[thomasanderson]

Wow formatting looks awful in the CL description. Is there a way I can disable formatting for a paragraph?

include/filesystem

public:
 
   ~filesystem_error() override; // key function
 
   _LIBCPP_INLINE_VISIBILITY
   const char* what() const noexcept override {
     return __storage_->__what_.c_str();
   }
 
-  _LIBCPP_FUNC_VIS
   void __create_what(int __num_paths);
 
 private:
   struct _Storage {
     _LIBCPP_INLINE_VISIBILITY
     _Storage(const path& __p1, const path& __p2) : __p1_(__p1), __p2_(__p2) {}
 
     path __p1_;