Don't crash when method decl is not found

Summary: public
Not finding C++ method in decl map was considered impossible to happen.
It turns out it can happen because ast exporter wasn't exporting template
instantiations.

So instead, pick a default when method decl is not found. This will be right in
most cases and will allow us to analyze rest of the function without crashing
frontend

Reviewed By: dulmarod

Differential Revision: D2695653

fb-gh-sync-id: 4077c59

infer/src/clang/cTrans.ml

@@ -394,7 +394,7 @@ struct
     let is_instance_method =
       match CMethod_trans.method_signature_of_pointer decl_ptr with
       | Some ms -> CMethod_signature.ms_is_instance ms
-      | _ -> assert false in (* will happen for generated methods, shouldn't happen right now *)
+      | _ -> true in (* might happen for methods that are not exported yet (some templates). *)
     let extra_exps = if is_instance_method then (
         (* pre_trans_result.exps may contain expr for 'this' parameter:*)
         (* if it comes from CXXMemberCallExpr it will be there *)