This came out of the first Rustfix WG meeting.

One of the goals is to enable Rustfix tests for all UI tests that
trigger lints with `MachineApplicable` suggestions. In order to do that
we first want to create a tracking issue that lists all files with
missing `// run-rustfix` headers.

This PR adds a `--rustfix-coverage` flag to `./x.py` and compiletest to
list the files with the missing headers in `/tmp/rustfix_missing_coverage.txt`.

From that file we can create the tracking issue and at some point also
enforce the `// run-rustfix` flag on UI tests with `MachineApplicable`
lints.

Not all suggestions come from lints