Fix 22369 - Omit unreachable statements in std.concurrency

[MoonlightSentinel]

Instantiating these templates with noreturn values / callbacks causes
the versioned statements to become unreachable.

---

Includes #8265 to make the test case compilable

[dlang-bot]

Thanks for your pull request and interest in making D better, @MoonlightSentinel! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22369	normal	Unreachable statements in std.concurrency with noreturn values / callbacks

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8266"

[thewilsonator]

It seems to me that noreturn has the same stink as void in requiring these special cases

[MoonlightSentinel]

Not particulary happy about these static-if's. But i don't think noreturn is to blame for better diagnostics, it's just the general problem of the overzealous statement not reachable warning in generic code.

[RazvanN7]

This seems to have exposed an ICE.

[MoonlightSentinel]

This seems to have exposed an ICE.

One of them, but the blocker for this PR was fixed in dlang/dmd#13142.