Remove save-analysis.

Most tests involving save-analysis were removed, but I kept a few where
the `-Zsave-analysis` was an add-on to the main thing being tested,
rather than the main thing being tested.

For `x.py install`, the `rust-analysis` target has been removed.

For `x.py dist`, the `rust-analysis` target has been kept in a
degenerate form: it just produces a single file `reduced.json`
indicating that save-analysis has been removed. This is necessary for
rustup to keep working.

Closes #43606.