Fix Issue 19251 - Alias this does not get called on struct qualified type

[RazvanN7]

On this branch [1] alias this isn't checked if implicit conversions cannot be done.

[1] https://github.com/dlang/dmd/pull/8711/files#diff-0c1aa1ff99402eb87e11cc3fa1b80da0R7664

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	19251	normal	Alias this does not get called on struct qualified type

Testing this PR locally

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

dub fetch digger
dub run digger -- build "master + dmd#8711"