[FIRRTL] Handle either kind of path attribute in LowerClasses.

Sometimes the start of a hierarchical path is an InnerRefAttr, and
sometimes the path is just a single FlatSymbolRefAttr. Either way, we
just need the StringAttr refering to the module name. There is a
helper called `root` for this on HierPathOp. But at this point we are
constructing the path to build a HierPathOp, we don't already have
one. So, I've implemented this the same as how `root` is implemented.

lib/Dialect/FIRRTL/Transforms/LowerClasses.cpp

@@ -384,7 +384,11 @@ LogicalResult LowerClassesPass::processPaths(
       // We will plumb in the basepath from this module.
       StringAttr altBasePathModule;
       if (needsAltBasePath) {
-        altBasePathModule = cast<hw::InnerRefAttr>(path.front()).getModule();
+        altBasePathModule =
+            TypeSwitch<Attribute, StringAttr>(path.front())
+                .Case<FlatSymbolRefAttr>([](auto a) { return a.getAttr(); })
+                .Case<hw::InnerRefAttr>([](auto a) { return a.getModule(); });
+
         pathInfoTable.addAltBasePathRoot(altBasePathModule);
       }