[RemoteMirror][swift-inspect] Add a command to inspect the state of the concurrency runtime.

[mikeash]

Most of the new inspection logic is in Remote Mirror. New code in swift-inspect calls the new Remote Mirror functions and formats the resulting information for display.

Specific Remote Mirror changes:

• Add a call to check if a given metadata is an actor.
• Add calls to get information about actors and tasks.
• Add a readObj call to MemoryReader that combines the read and the cast, greatly simplifying code chasing pointers in the remote process.
• Add a generalized facility to the C shims that can allocate a temporary object that remains valid until at least the next call, which is used to return various temporary arrays from the new calls. Remove the existing lastString and lastChunks member variables in favor of this new facility.

Swift-inspect changes:

• Add a new dump-concurrency command.
• Add a new ConcurrencyDumper.swift file with the implementation. The dumper needs to do some additional work with the results from Remote Mirror to build up the task tree and this keeps it all organized.
• Extend Inspector to query the target's threads and fetch each thread's current task.

Concurrency runtime changes:

• Add _swift_concurrency_debug variables pointing to the various future adapters. Remote Mirror uses these to provide a better view of a tasks's resume pointer.

A note on testing: I've been testing this manually. Automatic tests are difficult because the remote process inspection calls require root. I plan to work around this by having a swift-inspect mode that can inspect itself without root, either using the current calls (which may be able to inspect the current process without special privileges) or having a separate implemenattion of Inspector that uses in-process calls. That's going to be a bunch more work, though, so I'll do that in a separate PR afterwards.

rdar://88493833

[mikeash]

Here's an example of what this tool prints: https://gist.github.com/mikeash/80750c2f26ee13e5e60fdbc65da2c8c1

[DougGregor]

Here's an example of what this tool prints: https://gist.github.com/mikeash/80750c2f26ee13e5e60fdbc65da2c8c1

This is AMAZING. It's going to be our best favoritist debugging tool for concurrency runtime issues.

[mikeash]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - fb9550c5dca814eb41a7caee8b452178693b7860

[mikeash]

@swift-ci please test

[al45tair]

A couple of little things to look at, but looking good.

[mikeash]

@al45tair Thanks much for the review, pushed fixes now.

[mikeash]

@swift-ci please smoke test