Fix lifetime dependence diagnostics on Void types.

Allow a dependence on Void to be considered immortal. This is the ultimate
override in cases where no other code pattern is supported yet.

(cherry picked from commit 36d2b5bee4)

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/LifetimeDependenceDiagnostics.swift

@@ -199,9 +199,16 @@ private struct DiagnoseDependence {
     if function.hasUnsafeNonEscapableResult {
       return .continueWalk
     }
-    // If the dependence scope is global, then it has immortal lifetime.
-    if case .global = dependence.scope {
+    // Check for immortal lifetime.
+    switch dependence.scope {
+    case .global:
       return .continueWalk
+    case let .unknown(value):
+      if value.type.isVoid {
+        return .continueWalk
+      }
+    default:
+      break
     }
     // Check that the parameter dependence for this result is the same
     // as the current dependence scope.