Adjust for salsa::Id::from_u32() being unsafe

This impacts our manual `salsa::Id` wrappers. I refactored them a bit to improve safety.

crates/proc-macro-api/src/legacy_protocol/msg/flat.rs

@@ -72,7 +72,9 @@ pub fn deserialize_span_data_index_map(map: &[u32]) -> SpanDataIndexMap {
                     ast_id: ErasedFileAstId::from_raw(ast_id),
                 },
                 range: TextRange::new(start.into(), end.into()),
-                ctx: SyntaxContext::from_u32(e),
+                // SAFETY: We only receive spans from the server. If someone mess up the communication UB can happen,
+                // but that will be their problem.
+                ctx: unsafe { SyntaxContext::from_u32(e) },
             }
         })
         .collect()