[silgen] Use Builtin.ImplicitActor instead of Optional<any Actor> to represent the implicit isolated parameter.

NOTE: We are not performing any bitmasking at all now. This is so that we can
transition the code base/tests to expect Builtin.ImplicitActor instead
of Optional<any Actor>.

NOTE: The actual test changes are in the next commit. I did this to make it
easier to review the changes.

This should not have any user visible changes.

include/swift/SIL/ConcurrencyUtils.h

@@ -0,0 +1,38 @@
+//===--- ConcurrencyUtils.h -----------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_SIL_CONCURRENCYUTILS_H
+#define SWIFT_SIL_CONCURRENCYUTILS_H
+
+#include "swift/SIL/SILType.h"
+
+namespace swift {
+
+class SILValue;
+class SILBuilder;
+class SILLocation;
+
+/// Clear the implicit isolated bits of value.
+///
+/// \p value must be Builtin.ImplicitActor
+///
+/// \p finalType if empty, we always return
+/// Builtin.ImplicitActor. Otherwise we bitcast to finalType after
+/// tieing the lifetime of the result to \p value.
+SILValue clearImplicitActorBits(SILBuilder &b, SILLocation loc, SILValue value,
+                                SILType finalType = {});
+
+SILValue setImplicitActorBits(SILBuilder &b, SILLocation loc, SILValue value);
+
+} // namespace swift
+
+#endif

include/swift/SIL/SILBuilder.h

@@ -1330,6 +1330,17 @@ public:
         forwardingOwnershipKind));
   }
 
+  /// Create an unchecked_value_cast when Ownership SSA is enabled and
+  /// unchecked_bitwise_cast otherwise.
+  ///
+  /// Intended to be used in utility code that needs to support both Ownership
+  /// SSA and non-Ownership SSA code.
+  SILValue emitUncheckedValueCast(SILLocation loc, SILValue op, SILType ty) {
+    if (hasOwnership())
+      return createUncheckedValueCast(loc, op, ty);
+    return createUncheckedBitwiseCast(loc, op, ty);
+  }
+
   RefToBridgeObjectInst *createRefToBridgeObject(SILLocation Loc, SILValue Ref,
                                                  SILValue Bits) {
     return createRefToBridgeObject(Loc, Ref, Bits, Ref->getOwnershipKind());