incorrect suggestion span for no-mangle-const-items lint

[zackmdavis]

error: const items should never be #[no_mangle]
 --> const.rs:1:14
  |
1 | #[no_mangle] pub const RAH: usize = 5;
  |              -----^^^^^^^^^^^^^^^^^^^^
  |              |
  |              help: try a static value: `pub static`
  |
  = note: #[deny(no_mangle_const_items)] on by default

should look more like

error: const items should never be #[no_mangle]
 --> const.rs:1:14
  |
1 | #[no_mangle] pub const RAH: usize = 5;
  |                  -----^^^^^^^^^^^^^^^^
  |                  |
  |                  help: try a static value: `static`
  |
  = note: #[deny(no_mangle_const_items)] on by default

due to faulty assumptions introduced in #45232

[zackmdavis]

Illustration of implications for RLS users:

[zackmdavis]

This sort of thing (correct suggestions at the front of item-definitions, where there could be a pub, pub(crate) or pub(in path) item visibility modifier, or not) would be easier if hir::Visibility included the span of the visibility modifier. Would it be OK to plumb that through? If not, we're reduced to text-manipulation shenanigans on the item-definition span snippet, which would be messy to get correct across all cases.