Make Copy unsafe to implement for ADTs with unsafe fields

As a rule, the application of `unsafe` to a declaration requires that use-sites
of that declaration also require `unsafe`. For example, a field declared
`unsafe` may only be read in the lexical context of an `unsafe` block.

For nearly all safe traits, the safety obligations of fields are explicitly
discharged when they are mentioned in method definitions. For example,
idiomatically implementing `Clone` (a safe trait) for a type with unsafe fields
will require `unsafe` to clone those fields.

Prior to this commit, `Copy` violated this rule. The trait is marked safe, and
although it has no explicit methods, its implementation permits reads of `Self`.

This commit resolves this by making `Copy` conditionally safe to implement. It
remains safe to implement for ADTs without unsafe fields, but unsafe to
implement for ADTs with unsafe fields.

Tracking: rust-lang#132922

Author: jswrenn

clippy_lints/src/needless_pass_by_value.rs

@@ -200,6 +200,7 @@ impl<'tcx> LateLintPass<'tcx> for NeedlessPassByValue {
                                 cx.param_env,
                                 ty,
                                 traits::ObligationCause::dummy_with_span(span),
+                                rustc_hir::Safety::Safe,
                             )
                             .is_ok()
                             {