[clang] [clang] Adjust TextDiagnostic style ranges for interesting source region (PR #164941)

[via cfe-commits]

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@@ -1508,8 +1544,9 @@ void TextDiagnostic::emitSnippet(StringRef SourceLine,
   // Print the source line one character at a time.
   bool PrintReversed = false;
   std::optional<llvm::raw_ostream::Colors> CurrentColor;
-  size_t I = 0;
+  size_t I = 0; // Bytes.
----------------
Sirraide wrote:

```suggestion
  Bytes I = 0;
```
Or is there some reason we can’t do this here? 

(side note: `StyleRange` should probably also be updated to use `Columns`/`Bytes` at some point)

https://github.com/llvm/llvm-project/pull/164941