[Expressions] Fix -Wreorder warning from r353149

Summary:
```
ClangExpressionDeclMap.cpp:72:60: error: field 'm_struct_vars' will be initialized after field 'm_ctx_obj' [-Werror,-Wreorder]
      m_result_delegate(result_delegate), m_parser_vars(), m_struct_vars(),
```

Reviewers: bkramer, aleksandr.urakov

Reviewed By: aleksandr.urakov

Subscribers: aleksandr.urakov

Differential Revision: https://reviews.llvm.org/D57742

llvm-svn: 353161