fix issue 20042 - __vector CTFE crashes the compiler

[ghost]

This fix prevents the crash and a proper error message is issued, but doesn't add the code to accept the assign from slice to vector. This means that if this PR gets accepted then the issue would have to be reopened and converted to "enhancement".

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20042	major	__vector CTFE crashes the compiler

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 "stable + dmd#10160"

[tsbockman]

This solution is fine by me; thanks for working on this.

Please have a look at issue 20041, as well - its symptom is even worse, but I suspect it has the same root cause. If this PR can fix both, a separate test case should be added for 20041, since its symptom is different.

[ghost]

The other issue is a 2.085 regression. This one crashed since much longer.

[ibuclaw]

There is no need to change formatting here.

Format fixed.