[clang][deps] Support single-file mode for all formats (#88764)

The `clang-scan-deps` tool can be used for fast scanning of batches of
compilation commands passed in via the `-compilation-database` option.
This gets awkward in our tests where we have to resort to using
`.in`/`.template` JSON files and running them through `sed` in order to
embed LIT's `%t` variable into them. However, most of our tests only
need to pass single compilation command, so this dance is entirely
unnecessary.

This patch makes sure the existing "per-file" mode (where the
compilation command is passed in-line after the `--` argument) works for
all output formats, not only `P1689`.