[Strict memory safety] Treat the implicit call to a 'defer' function as implicitly 'unsafe'

This eliminates stray warnings.

test/Unsafe/safe.swift

@@ -293,3 +293,18 @@ struct UnsafeContainingUnspecified {
   let a = unsafe x.getA()
   _ = a
 }
+
+extension Slice {
+  // Make sure we aren't diagnosing the 'defer' as unsafe.
+  public func withContiguousMutableStorageIfAvailable<R, Element>(
+    _ body: (_ buffer: inout UnsafeMutableBufferPointer<Element>) throws -> R
+  ) rethrows -> R? where Base == UnsafeMutableBufferPointer<Element> {
+    try unsafe base.withContiguousStorageIfAvailable { buffer in
+      let start = unsafe base.baseAddress?.advanced(by: startIndex)
+      var slice = unsafe UnsafeMutableBufferPointer(start: start, count: count)
+      defer {
+      }
+      return try unsafe body(&slice)
+    }
+  }
+}