[Syntax Breaking] Rename DefaultImpl to AutoImpl

DefaultImpl is a highly confusing name for what we now call auto impls,
as in `impl Send for ..`. The name auto impl is not formally decided
but for sanity anything is better than `DefaultImpl` which refers
neither to `default impl` nor to `impl Default`.

src/librustc/middle/dead.rs

@@ -562,7 +562,7 @@ impl<'a, 'tcx> Visitor<'tcx> for DeadVisitor<'a, 'tcx> {
                 hir::ItemStruct(..) |
                 hir::ItemUnion(..) |
                 hir::ItemTrait(..) |
-                hir::ItemDefaultImpl(..) |
+                hir::ItemAutoImpl(..) |
                 hir::ItemImpl(..) => self.tcx.sess.codemap().def_span(item.span),
                 _ => item.span,
             };

src/librustc/middle/reachable.rs

@@ -270,7 +270,7 @@ impl<'a, 'tcx> ReachableContext<'a, 'tcx> {
                     hir::ItemMod(..) | hir::ItemForeignMod(..) |
                     hir::ItemImpl(..) | hir::ItemTrait(..) |
                     hir::ItemStruct(..) | hir::ItemEnum(..) |
-                    hir::ItemUnion(..) | hir::ItemDefaultImpl(..) |
+                    hir::ItemUnion(..) | hir::ItemAutoImpl(..) |
                     hir::ItemGlobalAsm(..) => {}
                 }
             }

src/librustc/middle/resolve_lifetime.rs

@@ -313,7 +313,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
             hir::ItemExternCrate(_) |
             hir::ItemUse(..) |
             hir::ItemMod(..) |
-            hir::ItemDefaultImpl(..) |
+            hir::ItemAutoImpl(..) |
             hir::ItemForeignMod(..) |
             hir::ItemGlobalAsm(..) => {
                 // These sorts of items have no lifetime parameters at all.