Add `report_fatal_error` to `getTypeForScalarType` (#1722)

Functions like `getTypeForScalarType` that do a mapping from one set
of types to another should not fail, and if they do it
should be obvious to the developer that that function has an
unhandled case.

Instead of silently failing when encountering an unsupported type,
this commit adds a `report_fatal_error` at the end, similar to other
type translation functions in this file.

lib/Dialect/Torch/Utils/Utils.cpp

@@ -96,7 +96,7 @@ Type Torch::getTypeForScalarType(
   case torch_upstream::ScalarType::Char:
     return mlir::IntegerType::get(context, 8, signedness);
   default:
-    return Type();
+    llvm::report_fatal_error("unhandled type for getTypeForScalarType");
   }
 }