Fix issue 20756: Handle interface argument in AllImplicitConversionTargets.

[FeepingCreature]

Replaces #7453

ping @berni44 , @RazvanN7

This should probably go in 2.097 so we don't release an incomplete version of ImplicitConversionTargets a second time. Does that need master or stable?

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20756	enhancement	ImplicitConversionTargets ignores interface inheritance

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8016"

[RazvanN7]

Since #7881 targeted master, this should also target master.

ImplicitConversionTargets was deprecated.

[berni44]

Hey great! I didn't touch the non built-ins, because I know too little about this.

[FeepingCreature]

As far as I can see, the buildkite failure is spurious: there's no actual error, just deprecations. The build tries to set it to succeed despite deprecations, but it doesn't work.

[RazvanN7]

Indeed, for some reason, the build hangs at some point for no apparent reason.

I restarted buildkite and seems to be green now.