[clang] [clang-format] Support BraceWrapping.AfterNamespace with AllowShortNamespacesOnASingleLine (PR #123010)

[Owen Pan via cfe-commits]

================
@@ -688,8 +701,8 @@ class LineJoiner {
     if (L2.First->isNot(tok::r_brace) || L2.First->MustBreakBefore)
       return 0;
 
-    // If so, merge all three lines.
-    return 2;
+    // If so, merge all lines.
+    return 2 + OpeningBraceLineOffset;
----------------
owenca wrote:

```suggestion
    return LinesToBeMerged;
```

https://github.com/llvm/llvm-project/pull/123010