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.

Fixes #81614

compiler/rustc_errors/src/emitter.rs

@@ -645,7 +645,7 @@ impl EmitterWriter {
         margin: Margin,
     ) {
         // Tabs are assumed to have been replaced by spaces in calling code.
-        assert!(!source_string.contains('\t'));
+        debug_assert!(!source_string.contains('\t'));
         let line_len = source_string.len();
         // Create the source line we will highlight.
         let left = margin.left(line_len);