Non-multiple of four indentation in closure arg pattern

The following unformatted line:

fn f() {
    let y = x.into_iter().map(|AaaaaaAaaaaaaaaaa{bbbbbb_bbbb,ccc_ccccccc}|());
}

currently gets formatted as:

fn f() {
    let y = x.into_iter().map(
        |AaaaaaAaaaaaaaaaa {
             bbbbbb_bbbb,
             ccc_ccccccc,
         }| (),
    );
}

This has some lines with 13 and 9 space indentation, which contradicts the indentation convention in the style guide RFC.

Each level of indentation must be four spaces (that is, all indentation outside of string literals and comments must be a multiple of four).

Start by reproducing the reported formatting with the Rust snippet in this issue, then trace the rustfmt handling for closure argument patterns. Add a regression test for this input and verify that every generated indentation level is a multiple of four, matching the linked style guide convention.