[clang-format] Fix an invalid code generation in RemoveBracesLLVM

[owenpan]

Summary

Fixes https://github.com/llvm/llvm-project/issues/55706.

[curdeius]

LGTM. Good catch for this bug!

clang/lib/Format/UnwrappedLineParser.cpp

if (Style.RemoveBracesLLVM) {
       Kind = IfStmtKind::IfElse;
     }
     nextToken();
     handleAttributes();
     if (FormatTok->is(tok::l_brace)) {
       FormatTok->setFinalizedType(TT_ElseLBrace);
       ElseLeftBrace = FormatTok;
       CompoundStatementIndenter Indenter(this, Style, Line->Level);
-      if (parseBlock(/*MustBeDeclaration=*/false, /*AddLevels=*/1u,
-                     /*MunchSemi=*/true,
-                     KeepElseBraces) == IfStmtKind::IfOnly) {
-        Kind = IfStmtKind::IfElseIf;
+      const IfStmtKind ElseBlockKind =
+          parseBlock(/*MustBeDeclaration=*/false, /*AddLevels=*/1u,
+                     /*MunchSemi=*/true, KeepElseBraces);
+      if ((ElseBlockKind == IfStmtKind::IfOnly ||
+           ElseBlockKind == IfStmtKind::IfElseIf) &&
+          FormatTok->is(tok::kw_else)) {
+        KeepElseBraces = true;
       }
       addUnwrappedLine();
     } else if (FormatTok->is(tok::kw_if)) {
       const FormatToken *Previous = Tokens->getPreviousToken();
       assert(Previous);
       const bool IsPrecededByComment = Previous->is(tok::comment);
       if (IsPrecededByComment) {
         addUnwrappedLine();

clang/unittests/Format/FormatTest.cpp

verifyFormat("if (a) {\n"
                "      e;\n"
                "  }\n"
                "} else {\n"
                "  g;\n"
                "}",
                Style);
 
   verifyFormat("if (a)\n"
+               "  if (b)\n"
+               "    c;\n"
+               "  else {\n"
+               "    if (d)\n"
+               "      e;\n"
+               "  }\n"
+               "else\n"
+               "  f;",
+               Style);
+
+  verifyFormat("if (a)\n"
+               "  if (b)\n"
+               "    c;\n"
+               "  else {\n"
+               "    if (d)\n"
+               "      e;\n"
+               "    else if (f)\n"
+               "      g;\n"
+               "  }\n"
+               "else\n"
+               "  h;",
+               Style);
+
+  verifyFormat("if (a)\n"
                "  b;\n"
                "else if (c)\n"
                "  while (d)\n"
                "    e;\n"
                "// comment",
                "if (a)\n"
                "{\n"
                "  b;\n"