[clang-format] Fix a bug in RemoveBracesLLVM

[owenpan]

Summary

Remove the braces of an else block only if the r_brace of the block is followed by an if.

clang/lib/Format/UnwrappedLineParser.cpp

FormatToken *UnwrappedLineParser::parseIfThenElse(IfStmtKind *IfKind,
   if (FormatTok->is(tok::kw_else)) {
     if (Style.RemoveBracesLLVM) {
       NestedTooDeep.back() = false;
       Kind = IfStmtKind::IfElse;
     }
     nextToken();
     handleAttributes();
     if (FormatTok->is(tok::l_brace)) {
+      const bool FollowedByIf = Tokens->peekNextToken()->is(tok::kw_if);
       FormatTok->setFinalizedType(TT_ElseLBrace);
       ElseLeftBrace = FormatTok;
       CompoundStatementIndenter Indenter(this, Style, Line->Level);
       IfStmtKind ElseBlockKind = IfStmtKind::NotIf;
       FormatToken *IfLBrace =
           parseBlock(/*MustBeDeclaration=*/false, /*AddLevels=*/1u,
                      /*MunchSemi=*/true, KeepElseBraces, &ElseBlockKind);
       if (FormatTok->is(tok::kw_else)) {
         KeepElseBraces = KeepElseBraces ||
                          ElseBlockKind == IfStmtKind::IfOnly ||
                          ElseBlockKind == IfStmtKind::IfElseIf;
-      } else if (IfLBrace && !IfLBrace->Optional) {
+      } else if (FollowedByIf && IfLBrace && !IfLBrace->Optional) {
         KeepElseBraces = true;
         assert(ElseLeftBrace->MatchingParen);
         markOptionalBraces(ElseLeftBrace);
       }
       addUnwrappedLine();
     } else if (FormatTok->is(tok::kw_if)) {
       const FormatToken *Previous = Tokens->getPreviousToken();
       assert(Previous);

clang/unittests/Format/FormatTest.cpp

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