fix ICE due to untranslated inner items in unreachable code

This code produces an ICE:

```rust
#![crate_type = "rlib"]
fn main() {
    if true { return }
    // remaining code is unreachable
    match () {
        () => { static MAGIC: uint = 0; }
    }
}
```
([playpen](http://is.gd/iwOISB))

The error is "encode_symbol: id not found 18", where 18 is the `NodeId` of the declaration of `MAGIC`.  The problem is that `rustc` tries to emit metadata for `MAGIC`, but some of the information is missing because `MAGIC` never gets translated by `trans_item` - the entire body of the `match` gets skipped because the `match` itself is unreachable.

This branch simplifies the handling of inner items by always processing them using the `trans_item` visitor, instead of sometimes using the visitor and sometimes waiting until `trans_stmt` encounters the item.  This fixes the ICE by making the translation of the item no longer depend on the declaration being reachable code.  This branch also reverts #16059 and #16359, since the new change to item translation fixes the same problems as those but is simpler.