[6.2] Fix LifetimeDependenceDiagnostics: allow inout assignment to Void.

[atrick]

Bypess lifetime dependence diagnostics completely for immortal values. We did
not do this initially because we wanted to potentially consider a value with a missing
dependency to mean that it could not escape the current function. But now we use
Void as a stand-in for immortal values.

This is needed for reassigning a Span/MutableSpan to an empty, immortal
Span:

func inoutToImmortal(_ s: inout RawSpan) {
  let tmp = RawSpan(_unsafeBytes: UnsafeRawBufferPointer(start: nil, count: 0))
  s = _overrideLifetime(tmp, borrowing: ())
}

Fixes rdar://152572002 ([GH:#81976] Cannot reinitialize inout parameter of type
MutableSpan<T>?)

(cherry picked from commit ac94d7d)

--- CCC ---

Explanation: Fix lifetime diagnostics to allow assigning an immortal value to an inout parameter.

Scope: This only affects adopters of experimental but supported @_lifetime syntax. The following example would incorrectly report a lifetime error:

func inoutToImmortal(_ s: inout RawSpan) {
  let tmp = RawSpan(_unsafeBytes: UnsafeRawBufferPointer(start: nil, count: 0))
  s = _overrideLifetime(tmp, borrowing: ())
}

Radar/SR Issue: rdar://152572002 ([GH:#81976] Cannot reinitialize inout parameter of type MutableSpan<T>?)

main PR: #82831

Risk: Low. No new logic is introduced. The existing check for immortal values is moved earlier to handle inout parameters as well as return value. The risk is that diagnostics are less restrictive.

Testing: Added source-level unit test.

Reviewer: TBD

[atrick]

@swift-ci test

[atrick]

@swift-ci test

[atrick]

Deciding not to cherry-pick this. There are workarounds for the known cases and we should be cautious about making diagnostics more permissive.