Don't fallback to copy + delete in rename.

Rename's documentation says "Files are renamed as if by POSIX rename()". and it
is used for atomically updating output files from a temporary. Having rename
fallback to a non atomic copy has the potential to hide bugs, like using
a temporary file in /tmp instead of a unique name next to the final destination.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186483 91177308-0d34-0410-b5e6-96231b3b80d8