cpp runtime cmake: add -Wno-overloaded-virtual to any target that includes generated headers

[jpy794]

Currently antlr4-generator cmake package only adds -Wno-overloaded-virtual flag to lexer / parser obj, but not to the target that includes generated headers. Since both gcc and clang add this flag to -Wall, it seems to be reasonable to handle this in antlr4-generator package.

Maybe we could create a target in antlr4-generator so that we could add something like this to solve the problem? Just a wild guess.

# antlr4-generator-config.cmake
# ...
target_compile_options(
    target_name
    PUBLIC -Wno-overloaded-virtual
)
# ...

I don't know if there's any plan to remove this flag from antlr4 cpp runtime, which should also be a good solution.