Harden copy_file even more.

This patch removes the O_CREAT open flag when we first
attempt to open the destination file but we expect it to
already exist.

This theoretically avoids the possibility that it was removed
between when we first stat'ed it, and when we attempt to open it.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@337659 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed