When SourceKit-LSP is shut down, we should make sure that we don’t leave behind child processes, which will become orphans after SourceKit-LSP has terminated. What’s worse, when SourceKit-LSP has exited, these processes might not have any process to read their stdout/stderr, which can lead to them running indefinitely.
This change does not cover the termination of subprocess trees. For example, if we launch `swift build` and need to kill it because it doesn’t honor SIGINT, its child processes will still live on. Similarly, if we kill a BSP server, its child processes might live on. Fixing this is a drastically bigger endeavor, likely requiring changes to Foundation and/or TSC. I filed https://github.com/swiftlang/sourcekit-lsp/issues/2080 for it.
