Misleading Diagnostics for Actors in Protocol Compositions

### Motivation Consider the following program that erroneously uses protocol composition for actor types `A` and `B`: ```swift actor A {} actor B {} typealias C = A & B // ❌ error: protocol-constrained type cannot contain class 'B' because it already contains class 'A' ``` The generated diagnostic is quite confusing. According to my understanding, actors simply don't support inheritance at all. Moreover, the following surprisingly compiles: ```swift actor A {} typealias B = A & CustomStringConvertible // ✅ ``` We only get an error when adding: ```swift await A().description // ❌ error: value of type 'A' has no member 'description' ``` ### Proposed solution I would expect the same message we get for structs, enums, functions and tuples: ``` ❌ error: non-protocol, non-class type 'A' cannot be used within a protocol-constrained type ``` ### Alternatives considered _No response_ ### Additional information swiftc --version: ``` Swift version 6.5-dev (LLVM 18d2bfb70c14d89, Swift c13d82e5987aecb) Target: x86_64-unknown-linux-gnu Build config: +assertions Compiler returned: 0 ```