Reduce tab formatting assertions to debug only
The tab replacement for diagnostics added in #79757 included a few assertions to ensure all tab characters are handled appropriately. We've started getting reports of these assertions firing (#81614). Since it's only a cosmetic issue, this downgrades the assertions to debug only, so we at least continue compiling even if the diagnostics might be a tad wonky.
Minimizes the impact of #81614
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