[libc++] Fix UB in filesystem::__copy for non-existent destination. (#87615)

The lstat/stat/fstat functions have no guarantee whether the `struct stat`
buffer is changed or not on failure. The filesystem::__copy function assumes
that the `struct stat` buffer is not updated on failure, which is not
necessarily correct.

It appears that for a non-existing destination `detail::posix_lstat(to,
t_st, &m_ec1)` returns a failure indicator and overwrites the `struct stat`
buffer with a garbage value, which is accidentally equal to the `f_st` from
stack internals from the previous `detail::posix_lstat(from, f_st, &m_ec1)` 
call.

file_type::not_found is a known status, so checking against 
`if (not status_known(t))` passes spuriously and execution continues.
Then the __copy function returns errc::function_not_supported because stats
are accidentally equivalent, which is incorrect.

Before checking for `detail::stat_equivalent`, we instead need to make sure 
that the call to lstat/stat/fstat was successful.

As a result of `f_st` and `t_st` not being accessed anymore without checking 
for the lstat/stat/fstat success indicator, it is not needed to zero-initialize 
them.

Author: ivafanas

libcxx/src/filesystem/operations.cpp

@@ -109,20 +109,20 @@ void __copy(const path& from, const path& to, copy_options options, error_code*
   const bool sym_status2 = bool(options & copy_options::copy_symlinks);
 
   error_code m_ec1;
-  StatT f_st = {};
+  StatT f_st;
   const file_status f =
       sym_status || sym_status2 ? detail::posix_lstat(from, f_st, &m_ec1) : detail::posix_stat(from, f_st, &m_ec1);
   if (m_ec1)
     return err.report(m_ec1);
 
-  StatT t_st          = {};
+  StatT t_st;
   const file_status t = sym_status ? detail::posix_lstat(to, t_st, &m_ec1) : detail::posix_stat(to, t_st, &m_ec1);
 
   if (not status_known(t))
     return err.report(m_ec1);
 
   if (!exists(f) || is_other(f) || is_other(t) || (is_directory(f) && is_regular_file(t)) ||
-      detail::stat_equivalent(f_st, t_st)) {
+      (exists(t) && detail::stat_equivalent(f_st, t_st))) {
     return err.report(errc::function_not_supported);
   }