[sledge] Classify issues translating type sizes as `todo` instead of `fail`

Summary:
If these failures are observed in real code of interest, they will
need to be resolved, so they are `todo` rather than an internal
`fail`ure.

Reviewed By: jvillard

Differential Revision: D24746222

fbshipit-source-id: 6b924be58

sledge/cli/frontend.ml

@@ -224,8 +224,8 @@ let size_of, bit_size_of =
     if Llvm.type_is_sized llt then
       match Int64.unsigned_to_int (size_of llt x.lldatalayout) with
       | Some n -> n
-      | None -> fail "type size too large: %a" pp_lltype llt ()
-    else fail "types with undetermined size: %a" pp_lltype llt ()
+      | None -> todo "type size too large: %a" pp_lltype llt ()
+    else todo "types with undetermined size: %a" pp_lltype llt ()
   in
   ( size_to_int Llvm_target.DataLayout.abi_size
   , size_to_int Llvm_target.DataLayout.size_in_bits )