Clang-CXX: copy into the dyndep output on success

When `clang-scan-deps` fails to scan (e.g., bad source syntax, junk
flags, etc.), the redirection unconditionally updates the file. If this
fails, the `.ddi` file timestamp is updated. If the state is then
reverted (e.g., the command line returns to the state of the last
successful build), the updated file is not useful, but `ninja` does not
rerun because:

- the command hash matches the last successful run
- the output file is newer than its inputs

However, since the `.ddi` file has been updated with bogus contents from
a failed scan, collation fails as the `rules` array is empty (or
incomplete from a batch scan).

If `clang-scan-deps` were properly aware of its output file, it could
use this to not write the file if any inner scan fails. Requested in
https://github.com/llvm/llvm-project/issues/72875.

See: https://github.com/llvm/llvm-project/issues/72875
Fixes: #25429

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