[Moore] Handle top-level typedefs by ignoring them

Typedefs are handled by Slang during type checking and don't need to be
lowered into anything in the IR. Therefore ignore top-level typedefs.

lib/Conversion/ImportVerilog/Structure.cpp

@@ -431,6 +431,13 @@ Context::convertCompilation(slang::ast::Compilation &compilation) {
   // which are listed separately as top instances.
   for (auto *unit : root.compilationUnits) {
     for (const auto &member : unit->members()) {
+      // Ignore top-level constructs that are handled by Slang as part of name
+      // resolution and type checking.
+      if (member.as_if<slang::ast::EmptyMemberSymbol>() ||
+          member.as_if<slang::ast::TransparentMemberSymbol>() ||
+          member.as_if<slang::ast::TypeAliasType>())
+        continue;
+
       // Error out on all top-level declarations.
       auto loc = convertLocation(member.location);
       return mlir::emitError(loc, "unsupported construct: ")

test/Conversion/ImportVerilog/basic.sv

@@ -4,6 +4,16 @@
 // Internal issue in Slang v3 about jump depending on uninitialised value.
 // UNSUPPORTED: valgrind
 
+// Ignore time unit and precision.
+timeunit 100ps;
+timeprecision 10fs;
+timeunit 100ps/10fs;
+`timescale 100ps/10fs;
+
+// Ignore type aliases and enum variant names imported into the parent scope.
+typedef int MyInt;
+typedef enum { VariantA, VariantB } MyEnum;
+
 // CHECK-LABEL: moore.module @Empty() {
 // CHECK:       }
 module Empty;