Fixing warning on MSVC. (#7567)

diff --git a/llvm-external-projects/iree-dialects/lib/Dialect/IREEPyDM/IR/Dialect.cpp b/llvm-external-projects/iree-dialects/lib/Dialect/IREEPyDM/IR/Dialect.cpp
index eb77c37..af9e341 100644
--- a/llvm-external-projects/iree-dialects/lib/Dialect/IREEPyDM/IR/Dialect.cpp
+++ b/llvm-external-projects/iree-dialects/lib/Dialect/IREEPyDM/IR/Dialect.cpp
@@ -215,6 +215,9 @@
       assert(getUniformElementType() &&
              "unboxed list should have uniform element type");
       return getUniformElementType();
+    default:
+      llvm_unreachable("unsupported storage class");
+      return {};
   }
 }