fix(forms): FormControlName directive not cleaning up on destroy

[crisbeto]

Fixes a memory leak caused by the FormControlName directive which wasn't cleaning up after itself on destroy. It ended up retaining a reference to the detached DOM node that the directive is applied to.

Fixes #37431.

[AndrewKushnir]

Hi @crisbeto, thanks for creating this PR.

I was working on that problem as a part of #37566 (it's in a Draft mode, since I was checking how things work with Google apps). This part of the fix from #37566 is very similar to what you implemented (which confirms the overall direction), but the idea that I had was to keep the setUpControl and cleanUpControl (which performs reverse operation from setUpControl) function calls on the same logical level (i.e. within the same class). Also it turned out that the cleanUpControl function itself is not working correctly in all cases (see additional changes in #37566), for example it doesn't clear validators attached to the FormControl instances - I plan to fix that too after landing #37881 (which should allow more granular operations with the list of validators). The delta between our fixes is that you also clear the control field and I should look into that, so let's keep this PR open and I will share more info after additional investigation.

Thank you.

[crisbeto]

Closing in favor of #37566.

[rppig42]

Hi there, would this patch be merged into Angular@9.x?

And thank you for your work

[crisbeto]

It might be better to ask on #37566 since this PR didn't end up getting merged.

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