Add arg_count field to Body in Stable MIR

This field allows SMIR consumers to identify which locals correspond to
argument locals. It simply exposes the arg_count field from the MIR
representation.

compiler/rustc_smir/src/rustc_smir/mod.rs

@@ -308,6 +308,7 @@ impl<'tcx> Stable<'tcx> for mir::Body<'tcx> {
                     span: decl.source_info.span.stable(tables),
                 })
                 .collect(),
+            arg_count: self.arg_count,
         }
     }
 }