Fix LifetimeDependenceDiagnostics: allow inout assignment to Void.

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:swiftlang#81976] Cannot reinitialize inout parameter of type
`MutableSpan<T>?`)

Author: atrick

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/LifetimeDependenceDiagnostics.swift

@@ -110,6 +110,20 @@ private func analyze(dependence: LifetimeDependence, _ context: FunctionPassCont
     }
   }
 
+  // Check for immortal dependence.
+  switch dependence.scope {
+  case .global:
+    log("Immortal global dependence.")
+    return true
+  case let .unknown(value):
+    if value.type.isVoid {
+      log("Immortal void dependence.")
+      return true
+    }
+  default:
+    break
+  }
+
   // Compute this dependence scope.
   var range = dependence.computeRange(context)
   defer { range?.deinitialize() }
@@ -200,6 +214,9 @@ private struct DiagnoseDependence {
       return .continueWalk
     }
     // Check for immortal lifetime.
+    //
+    // FIXME: remove this immortal check. It should be redundant with the earlier check that bypasses dependence
+    // diagnostics.
     switch dependence.scope {
     case .global:
       return .continueWalk

test/SILOptimizer/lifetime_dependence/verify_diagnostics.swift

@@ -287,3 +287,13 @@ func testSpanMayThrow(buffer: inout [Int]) {
   let bufferSpan = buffer.mutableSpan
   try! mutableSpanMayThrow(bufferSpan)
 }
+
+// =============================================================================
+// inout
+// =============================================================================
+
+@available(Span 0.1, *)
+func inoutToImmortal(_ s: inout RawSpan) {
+  let tmp = RawSpan(_unsafeBytes: UnsafeRawBufferPointer(start: nil, count: 0))
+  s = _overrideLifetime(tmp, borrowing: ())
+}