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.
Fixes#81614
This replaces tabs earlier in the diagnostics emitting process, which allows
various margin calculations to ignore the existence of tabs. It does add a
string copy for the source lines that are emitted.
