[clang] [clang-format] Propagate `LeadingEmptyLinesAffected` when joining lines (PR #146761)

[Owen Pan via cfe-commits]

================
@@ -986,7 +986,7 @@ class LineJoiner {
   void join(AnnotatedLine &A, const AnnotatedLine &B) {
     assert(!A.Last->Next);
     assert(!B.First->Previous);
-    if (B.Affected)
+    if (B.Affected || (B.LeadingEmptyLinesAffected && A.Last->Children.empty()))
----------------
owenca wrote:

Either dropping the condition with a comment, or adding an assertion would be fine by me. For example,
```cc
    if (B.Affected || B.LeadingEmptyLinesAffected) {
      assert(B.Affected || A.Last->Children.empty());
```

https://github.com/llvm/llvm-project/pull/146761