Resolve empty statement warning when using PYBIND11_OVERLOAD_PURE_NAME and PYBIND11_OVERLOAD_PURE (#2325)
* Wrap PYBIND11_OVERLOAD_NAME and PYBIND11_OVERLOAD_PURE_NAME in do { ... } while (false), and resolve trailing semicolon
* Deprecate PYBIND11_OVERLOAD_* and get_overload in favor of PYBIND11_OVERRIDE_* and get_override
* Correct erroneous usage of 'overload' instead of 'override' in the implementation and internals
* Fix tests to use non-deprecated PYBIND11_OVERRIDE_* macros
* Update docs to use override instead of overload where appropriate, and add warning about deprecated aliases
* Add semicolons to deprecated PYBIND11_OVERLOAD macros to match original behavior
* Remove deprecation of PYBIND11_OVERLOAD_* macros and get_overload
* Add note to changelog and upgrade guide
