Reinstate local definitions of _overrideLifetime()

This partially reverts swiftlang#1400

Author: glessard

Sources/FoundationEssentials/Data/Data.swift

@@ -2974,3 +2974,53 @@ extension Data : Codable {
         }
     }
 }
+
+// TODO: remove once _overrideLifetime is public in the standard library
+/// Unsafely discard any lifetime dependency on the `dependent` argument. Return
+/// a value identical to `dependent` with a lifetime dependency on the caller's
+/// borrow scope of the `source` argument.
+@unsafe
+@_unsafeNonescapableResult
+@_alwaysEmitIntoClient
+@_transparent
+@lifetime(borrow source)
+internal func _overrideLifetime<
+  T: ~Copyable & ~Escapable, U: ~Copyable & ~Escapable
+>(
+  _ dependent: consuming T, borrowing source: borrowing U
+) -> T {
+  dependent
+}
+
+/// Unsafely discard any lifetime dependency on the `dependent` argument. Return
+/// a value identical to `dependent` that inherits all lifetime dependencies from
+/// the `source` argument.
+@unsafe
+@_unsafeNonescapableResult
+@_alwaysEmitIntoClient
+@_transparent
+@lifetime(copy source)
+internal func _overrideLifetime<
+  T: ~Copyable & ~Escapable, U: ~Copyable & ~Escapable
+>(
+  _ dependent: consuming T, copying source: borrowing U
+) -> T {
+  dependent
+}
+
+/// Unsafely discard any lifetime dependency on the `dependent` argument.
+/// Return a value identical to `dependent` with a lifetime dependency
+/// on the caller's exclusive borrow scope of the `source` argument.
+@unsafe
+@_unsafeNonescapableResult
+@_alwaysEmitIntoClient
+@_transparent
+@lifetime(&source)
+internal func _overrideLifetime<
+  T: ~Copyable & ~Escapable, U: ~Copyable & ~Escapable
+>(
+  _ dependent: consuming T,
+  mutating source: inout U
+) -> T {
+  dependent
+}