Remove the unused m_ast_context member of ClangExpressionParser.

[sivachandra]

Will commit this as obvious.

include/lldb/Expression/ClangExpressionParser.h

     
 private:
     ClangExpression &                       m_expr;                 ///< The expression to be parsed
     std::unique_ptr<llvm::LLVMContext>       m_llvm_context;         ///< The LLVM context to generate IR into
     std::unique_ptr<clang::FileManager>      m_file_manager;         ///< The Clang file manager object used by the compiler
     std::unique_ptr<clang::CompilerInstance> m_compiler;             ///< The Clang compiler used to parse expressions into IR
     std::unique_ptr<clang::Builtin::Context> m_builtin_context;      ///< Context for Clang built-ins
     std::unique_ptr<clang::SelectorTable>    m_selector_table;       ///< Selector table for Objective-C methods
-    std::unique_ptr<clang::ASTContext>       m_ast_context;          ///< The AST context used to hold types and names for the parser
     std::unique_ptr<clang::CodeGenerator>    m_code_generator;       ///< The Clang object that generates IR
     
     class LLDBPreprocessorCallbacks;
     LLDBPreprocessorCallbacks               *m_pp_callbacks;         ///< Called when the preprocessor encounters module imports
 };
     
 }
 
 #endif  // liblldb_ClangExpressionParser_h_