macOS 11 has been EoL for 2 years, and we'd like to shut down the macOS 11 distro. This requires getting 7.0 to build on macOS 14 instead. XCode on macOS uses a more recent version of Clang that we use on later branches, so we need to update the C++ code for compatibility.

Preliminary investigation indicates that this primarily requires fixing some issues with forward declarations of C++ classes. Specifically the only main issue is that vector instantiates need their types to be complete in more scenarios.

Prior issues to address this include SERVER-79426, which was never backported.