[Console] Add clear() to ProgressHelper.

One may want to print something else while the progress bar is running.
The output will be messy if the progress bar is not removed first.

One may also want to remove the progress bar after the work is complete.
merged branch mpartel/progresshelper-clear (PR #8074)

This PR was merged into the master branch.

Discussion
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[Console] Add clear() to ProgressHelper.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        | symfony/symfony-docs#2642

One may want to print something else while the progress bar is running.
The output will be messy if the progress bar is not removed first.

One may also want to remove the progress bar after the work is complete.

Commits
-------

29c71a5 [Console] Add clear() to ProgressHelper.