fix ICE due to untranslated inner items in unreachable code

[spernsteiner]

This code produces an ICE:

#![crate_type = "rlib"]
fn main() {
    if true { return }
    // remaining code is unreachable
    match () {
        () => { static MAGIC: uint = 0; }
    }
}

(playpen)

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.

[steveklabnik]

This PR has some kind of conflict, and can't be merged. It'll need a rebase.

[spernsteiner]

OK, rebased it.