[CFE] Allow coverage tool up update comments and expectation file in one go

Previously to update comments "correctly" one had to
1) Do a coverage run with debug enabled to save the coverage files.
2) Pass the coverage files into the comment removal tool.
3) Do another coverage run with debug enabled to save the coverage files
   (for the now changed sources).
4) Pass the coverage files into the comment adding tool.
5) Do a third coverage run, updating the expectation (hopefully, saying
   0 missis in all files).
6) Format the changed files.

Now instead one can run

```
out/ReleaseX64/dart-sdk/bin/dart pkg/front_end/test/coverage_suite.dart --add-and-remove-comments --update-expectations
```

and it will all be done with only 1 run.

This CL also adds support for adding coverage igonre comments to `a?[b]`
where `a` is null and the lookup (and `b`) is thus not performed.
This should let the misses go to 0 (in a followup CL).

Change-Id: Ib96f3568f201586efe6f2d178133485d56a28be0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/378440
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>