[5.9] SE-0365 bug fixes

[calda]

This PR cherry picks two bug fixes for SE-0365 into the Swift 5.9 release branch.

• Explanation: This fixes two bugs in the implementation of SE-0365, where code that is expected to compile would unexpectedly result in an error (examples below).
• Scope: This fixes two bugs in the implementation of SE-0365, which originally landed in Swift 5.8.
• Issue: fixes issues described in [SE-0365] Fix issue where boolean condition before let self condition would prevent using implicit self #65310 / [SE-0365] Allow implicit self in inner functions in [weak self] closures, like with [self] closures #65211 and shown below (I didn't see or file an issue or radar for these issues, I just fixed them once I found them)
• Risk: These are targeted fixes than only affect the behavior of implicit self in weak self closures. There is little risk that this would cause a source compatibility issue, since these changes allow code that was previously rejected. It is also unlikely that this would cause a regression since there are pretty extensive unit tests for the expected behavior.
• Testing: CI test suite and source compatibility tests
• Reviewers: @DougGregor (5.9 release manager), @xedin

I CP'd both of these changes in the same PR because the implementation of #65310 depends on changes in #65211

---

Examples of code that is currently rejected in Swift 5.8 / 5.9 but compiles successfully with these changes as expected by SE-0365:

class Test {
  func method() { }
  
  func foo(condition: Bool) {
    doVoidStuff { [weak self] in
      guard condition, let self else { return }
      method() // 🛑 Unexpected error: call to method 'method' in closure requires explicit use of 'self' to make capture semantics explicit
    }
  }
}

doVoidStuff { [weak self] in
  guard let self else { return }
  func innerFunction() {
    // 🛑 Error: call to method 'test' in closure requires explicit use of 'self' to make capture semantics explicit
    test()
  }
}

[xedin]

@swift-ci please test

[hborla]

@swift-ci please test

[calda]

Looks like the Swift Test macOS Platform job failed in update-checkout. Smoke tests passed though.

[xedin]

@swift-ci please test macOS platform

[calda]

All green now, thanks @xedin