cpp runtime better not to define symbol in global namespace, like Ref

[byronhe]

antlr defined Ref in global namespace at runtime/Cpp/runtime/src/antlr4-common.h ,
this definition caused symbol conflict with my internal codebase.

in my opinion, as a library , antrl4-cpp-runtime should not define symbols in global namespace, a better way should be include Ref in namespace like antlrcpp.