Only apply NoMangle lint to plain const items

[reddevilmidzy]

resolve: #149511

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[RUSTC-TIMING] tracing_tree test:false 0.482
   Compiling rustc_borrowck v0.0.0 (/checkout/compiler/rustc_borrowck)
[RUSTC-TIMING] rustc_passes test:false 12.605
   Compiling rustc_resolve v0.0.0 (/checkout/compiler/rustc_resolve)
error[E0615]: attempted to take value of method `is_empty` on type `&[rustc_hir::GenericParam<'_>]`
    --> compiler/rustc_lint/src/builtin.rs:1002:40
     |
1002 |                     && generics.params.is_empty
     |                                        ^^^^^^^^ method, not a field
     |

[fmease]

(unrelated) This cx.tcx.sess.source_map().span_to_snippet(it.span).map(|snippet| snippet.find("const").unwrap_or(0)).unwrap_or(0) as u32; let suggestion = it.span.with_hi(BytePos(it.span.lo().0 + start + 5)); can probably be made more robust (accounting for source code comments for example) & simplified by changing it to let suggestion = item.vis_span.to(ident.span); (at least I think that's what it's computing).

[fmease]

We still want to emit the lint, we just want to suppress the suggestion. So make the suggestion in BuiltinConstNoMangle Optional and set it to None if there are params or a where-clause.

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[bjorn3]

This is not what this test file tests. Also please give the test a better name and location.