[clang] [clang-format] Add `AllowShortNamespacesOnASingleLine` option (PR #105597)

[Owen Pan via cfe-commits]

================
@@ -361,9 +361,19 @@ class LineJoiner {
     const auto *FirstNonComment = TheLine->getFirstNonComment();
     if (!FirstNonComment)
       return 0;
+
     // FIXME: There are probably cases where we should use FirstNonComment
     // instead of TheLine->First.
 
+    if (TheLine->Last->is(tok::l_brace)) {
+      if (Style.AllowShortNamespacesOnASingleLine &&
+          TheLine->First->is(tok::kw_namespace)) {
+        unsigned result = tryMergeNamespace(I, E, Limit);
+        if (result > 0)
+          return result;
+      }
+    }
----------------
owenca wrote:

```suggestion
    if (Style.AllowShortNamespacesOnASingleLine &&
        TheLine->First->is(tok::kw_namespace) &&
        TheLine->Last->is(tok::l_brace)) {
      const auto result = tryMergeNamespace(I, E, Limit);
      if (result > 0)
        return result;
    }
```

https://github.com/llvm/llvm-project/pull/105597