Fixup missing renames from #[main] to #[rustc_main]

In fc357039f9 `#[main]` was removed and replaced with `#[rustc_main]`.
In some place the rename was forgotten, which makes the current code
confusing, because at first glance it seems that `#[main]` is still
around. Perform the renames also in these places.

compiler/rustc_ast/src/entry.rs

@@ -2,7 +2,7 @@
 pub enum EntryPointType {
     None,
     MainNamed,
-    MainAttr,
+    RustcMainAttr,
     Start,
     OtherMain, // Not an entry point, but some other function named main
 }