sumtype: work around issue 21975 in isSumType

[pbackus]

isSumType!T now evaluates to true instead of false when T is a templated
struct type that implicitly converts to a SumType via alias this.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✗	21975	normal	is expression ignores implicit conversion of struct via alias this when pattern matching

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#8394"