AFAICT there isn't a way to suppress these, since Swift doesn't really seem to support granular control over the errors & warnings produced by the compiler. these diagnostics appear come from SILGen. it seems fairly intentional that any unreachable source code produce a warning, so if there were a way to avoid these, it would likely indicate a bug.
not a very satisfying answer to the issue at hand... it would seem best if the code generator could be aware of the 'inhabitedness' of the type to just conditionally avoid generation, but idk how realistic that change may be.