CppEditor: Add support for init statements in if conditions

Fixes: QTCREATORBUG-29182
Change-Id: I9b7969da694b368236246123ad0028d8e754e903
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/libs/3rdparty/cplusplus/AST.h

@@ -1779,6 +1779,8 @@ public:
     int if_token = 0;
     int constexpr_token = 0;
     int lparen_token = 0;
+    DeclarationAST *initDecl = nullptr;
+    StatementAST *initStmt = nullptr;
     ExpressionAST *condition = nullptr;
     int rparen_token = 0;
     StatementAST *statement = nullptr;