Script to Coalesce Output From Multiple Platforms

[dsanders11]

One of the shortcomings of clangd with this approach is the output is pretty platform-specific, which means it's not a complete picture, and it may suggest removing includes which are used on other platforms, because it is not used on the platform it is being run on.

A way to minimize the impact of that would be to take output from multiple platforms (Win, Linux, macOS, etc) and coalesce them into a single output.

The script should take N output files and coalesce them, and should have options for either doing a union or an intersection (default) of the outputs.