Fix 21779 - Don't generate messages for disabled assertions

[MoonlightSentinel]

Only generate messages when the assert is not omitted due to -release or -check=assert=off.
The rewrite to create a temporary seems to prevent the ellision (and is a waste of time anyways).

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21779	normal	assert not omitted for -release -checkaction=context

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#12315"