message: Add support for emitting functions with `coldcc` in LLVM
  The eventual goal is to try using this for things like the internal panicking stuff, to see whether it helps.
diff: @@ -22,7 +22,7 @@ fn clif_sig_from_fn_abi<'tcx>(
  fn_abi: &FnAbi<'tcx, Ty<'tcx>>,
  ) -> Signature {
  let call_conv = match fn_abi.conv {
  - Conv::Rust | Conv::C => default_call_conv,
  + Conv::Rust | Conv::C | Conv::RustCold => default_call_conv,
  Conv::X86_64SysV => CallConv::SystemV,
  Conv::X86_64Win64 => CallConv::WindowsFastcall,
  Conv::ArmAapcs
  