Added ObjC protocol expressions.

src/shared/cplusplus/AST.cpp

src/shared/cplusplus/AST.h

src/shared/cplusplus/ASTClone.cpp

src/shared/cplusplus/ASTVisit.cpp

src/shared/cplusplus/ASTVisitor.h

src/shared/cplusplus/ASTfwd.h

src/shared/cplusplus/Parser.cpp