message: Explain why we switch away from large code model when PIC is enabled.
  Note that I don't explain why Small vs Medium. I'm not sure myself.
diff: @@ -95,6 +95,11 @@ impl LLVM {
  }
  fn code_model(&self) -> CodeModel {
  + // We normally use the large code model, but when targeting shared
  + // objects, we are required to use PIC. If we use PIC anyways, we lose
  + // any benefit from large code model and there's some cost on all
  + // platforms, plus some platforms (MachO) don't support PIC + large
  + // at all.
  if self.is_pic {
  CodeModel::Small
  } else {
  