Fix Issue 21039 - alias this returns 'null' for ref types when put into array initializer

[BorisCarvajal]

Fix typeMerge() to use implicit conversion instead of explicit cast on classes to allow going through alias this.

[dlang-bot]

Thanks for your pull request, @BorisCarvajal!

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21039	regression	`alias this` returns 'null' for ref types when put into array initializer

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 "stable + dmd#13277"