keep hmr update with output.clean=true

[vankop]

What kind of change does this PR introduce?

fixes #15206

Did you add tests for your changes?

yes

Does this PR introduce a breaking change?
no

What needs to be documented once your changes are merged?
nothing

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[webpack-bot]

@vankop Thanks for your update.

I labeled the Pull Request so reviewers will review it again.

@sokra Please review the new changes.

[sokra]

Thanks