Fix #7740: gather_loans should not recur into the items of the block.

gather_loans does not need to recurse into any items declared in the
current block.  Rather than special-case `fk_item_fn` and `fk_method`,
just make the GatherLoanVisitor's visit_item method a no-op.

This indirectly implies that the example of #7740 is fixed:

    fn f() {
        static A: &'static char = &'A';
    }

Since we do not recurse into items, we no longer encounter `&'A'`.
regression test for #7740.
auto merge of #8993 : pnkfelix/rust/fsk-fix-7740-dont-recur-on-items-during-gather-loans-of-block, r=nikomatsakis

Fix #7740

r? anyone, @nikomatsakis especially.