[clang] [clang-format] Fix repeated backslash insertion in macro line comments (PR #164300)

[Björn Schäpers via cfe-commits]

================
@@ -798,9 +798,11 @@ void ContinuationIndenter::addTokenOnCurrentLine(LineState &State, bool DryRun,
   }
 
   if (!DryRun) {
+    bool ContinuePPDirective =
----------------
HazardyKnusperkeks wrote:

```suggestion
    const bool ContinuePPDirective =
```

https://github.com/llvm/llvm-project/pull/164300