C++ exceptions should have a noexcept copy constructor and assignment operator

[bernardnormier]

Related to: zeroc-ice/ice-demos#146

All C++ local exceptions should follow this rule.

For generated user exceptions with user-specified fields, it should be true when the fields are noexcept copy-constructible and assignable. Unfortunately we can't provide this guarantee for user exceptions with fields that aren't noexcept copy-constructible (like std::string) without breaking source compatibility. We could try to disable this diagnostics for such generated user exceptions.