Use _LIBCPP_UNREACHABLE to convince GCC that non-void functions actually always return

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@337519 91177308-0d34-0410-b5e6-96231b3b80d8

src/experimental/filesystem/filesystem_common.h

@@ -228,8 +228,8 @@ public:
 } // namespace time_util
 
 
-using TimeSpec = struct timespec;
-using StatT = struct stat;
+using TimeSpec = struct ::timespec;
+using StatT = struct ::stat;
 
 using FSTime = time_util::fs_time_util<file_time_type, time_t, struct timespec>;