Fix 21752 - Template constraint breaks nested eponymeous template

[MoonlightSentinel]

The callsc can be identical for deeply nested eponymeous templates or
while doing overload resolution, so ignore the failure if it's the first
checked scope.

CC @rainers because this modifies the check from #10239

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21752	regression	Template constraint breaks nested eponymeous template

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 + dmd#12306"