[6.0] Concurrency: Minor fixes for isolation checking

[slavapestov]

• Description: Adding some new assertions on main exposed bugs where isolation checking was looking at captures before they were computed, or before they could be computed because the AST was not yet type checked. Other than doing repeated work from visiting children unnecessarily, the only real issue here was that if you had a closure nested inside of another closure inside a stored property initializer, we didn't diagnose non-Sendable captures in -swift-version 6 mode.
• Scope of the issue: I discovered these problems myself while fixing on something else but it would've come up eventually.
• Tested: There's a new test case that used to work but wasn't covered by existing tests (top-level computed property in script mode) and a test case that did not diagnose correctly (the nested closure in initializer thing).
• Risk: Low.
• Reviewed by: @hborla

[slavapestov]

@swift-ci Please test

[xavgru12]

I went through the logs of the failed step and found:

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hudson.AbortException: script returned exit code 1
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Where exactly did it fail?

[slavapestov]

@swift-ci Please test macOS