rollup merge of rust-lang#20027: michaelwoerister/for-loop-var

Back when for-loop iteration variables were just de-sugared into `let` bindings, debuginfo for them was created like for any other `let` binding. When the implementation approach for for-loops changed, we ceased having debuginfo for the iteration variable. This PR fixes this omission and adds a more prominent test case for it.

Also contains some minor, general cleanup of the debuginfo module.

Fixes rust-lang#19732

Author: alexcrichton

src/librustc_trans/trans/controlflow.rs

@@ -286,6 +286,7 @@ pub fn trans_for<'blk, 'tcx>(mut bcx: Block<'blk, 'tcx>,
     debug!("iterator type is {}, datum type is {}",
            ppaux::ty_to_string(bcx.tcx(), iterator_type),
            ppaux::ty_to_string(bcx.tcx(), iterator_datum.ty));
+
     let lliterator = load_ty(bcx, iterator_datum.val, iterator_datum.ty);
 
     // Create our basic blocks and set up our loop cleanups.
@@ -365,6 +366,8 @@ pub fn trans_for<'blk, 'tcx>(mut bcx: Block<'blk, 'tcx>,
                                        llpayload,
                                        binding_cleanup_scope_id);
 
+    debuginfo::create_for_loop_var_metadata(body_bcx_in, pat);
+
     // Codegen the body.
     body_bcx_out = trans_block(body_bcx_out, body, expr::Ignore);
     body_bcx_out =