Check proc_macro attributes on statement items

[csmoe]

Closes #52329
r? @petrochenkov

[petrochenkov]

@csmoe
There are many locations that the check_attr infra doesn't check.
Here's a list collected from hir/mod.rs:

• generic parameter (GenericParam)
• let statement (Local)
• match arm (Arm)
• expression (Expr)
• trait item (TraitItem)
• impl item (ImplItem)
• enum variant (Variant_)
• struct field (StructField)
• item (Item)
• foreign item (ForeignItem)
• crate root (Crate)

From these locations only items, expressions and let statements are checked.
So, with this PR as is, proc-macro attributes are checked for items (in proc_macro_registrar) and for let statements (check_attr). Doesn't look like a noticeable improvement.

I think we'll need to build full attr checking infra eventually, but it doesn't seem to worth it for proc-macro attributes alone.
If anything, we can start reporting an unused attribute lint for proc-macro attributes in wrong positions at any time.

I'll close this PR and #52329 as well as wontfix.