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

[Owen Pan via cfe-commits]

================
@@ -628,28 +632,36 @@ class LineJoiner {
 
   unsigned tryMergeNamespace(ArrayRef<AnnotatedLine *>::const_iterator I,
                              ArrayRef<AnnotatedLine *>::const_iterator E,
-                             unsigned Limit) {
+                             unsigned Limit, bool OpenBraceWrapped) {
     if (Limit == 0)
       return 0;
 
-    assert(I[1]);
-    const auto &L1 = *I[1];
+    // The merging code is relative to the opening namespace brace, which could
+    // be either on the first or second line due to the brace wrapping rules.
+    const auto OpeningBraceLineOffset = OpenBraceWrapped ? 1 : 0;
+    const auto BraceOpenLine = I + OpeningBraceLineOffset;
----------------
owenca wrote:

```suggestion
    const bool OpenBraceWrapped = Style.BraceWrapping.AfterNamespace;
    const auto *BraceOpenLine = I + OpenBraceWrapped;

    assert(*BraceOpenLine);
    if ((*BraceOpenLine)->Last->isNot(tok::l_brace))
      return 0;
```

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