Make PassWhere impl Copy.

It's a very small and simple type.

compiler/rustc_borrowck/src/polonius/dump.rs

@@ -226,7 +226,7 @@ fn emit_polonius_mir<'tcx>(
         regioncx,
         closure_region_requirements,
         borrow_set,
-        pass_where.clone(),
+        pass_where,
         out,
     )?;
 

compiler/rustc_middle/src/mir/pretty.rs

@@ -22,7 +22,7 @@
 
 /// An indication of where we are in the control flow graph. Used for printing
 /// extra information in `dump_mir`
-#[derive(Clone)]
+#[derive(Clone, Copy)]
 pub enum PassWhere {
     /// We have not started dumping the control flow graph, but we are about to.
     BeforeCFG,