Enable strict concurrency checking in CI

[czechboy0]

Summary

To help avoid concurrency bugs, this PR enables complete concurrency checking mode and addresses all the current concurrency warnings.

Some notable changes:

• Moved some locking primitives into SPI of Instrumentation, to be usable in the rest of the package
• Created a LockedValueBox type, used in several places to protect mutable state
• added sendability annotations where it made sense
• introduced a small workaround for accessing stdout/stderr (which emitted a warning, as it's a read-write global variable), using fdopen, which I think should be fine, but calling it out
• reformatted using ./scripts/validate_format.sh, so might be worth reviewing the diff with ?w=1

Test Plan

Tested that all builds and tests pass on macOS and Linux.

[tomerd]

@swift-server-bot add to allowlist

[czechboy0]

Thanks for the review, @ktoso, will address your feedback!

[czechboy0]

@ktoso, ok I commented out the strict concurrency flags for now, but I think taking these changes would still be valuable. Once we have an answer for making withSpan safe, we can just uncomment the flags.

[ktoso]

It's a good change over all, I'll give it another skim and merge. Thank you @czechboy0 !

Related radars: rdar://113846321 rdar://113848512

[ktoso]

Looks good, thank you @czechboy0 !

And we'll follow up on those radars about strict checking and closures