[MooreToCore] Mark Seq dialect as legal

Add the Seq dialect as a legal dialect for the Moore-to-Core conversion.
This will allow the conversion to properly pass through `seq.to_clock`
in case the conversion is called on an IR that is already lowered to the
core dialects. This allows circt-verilog to accept core-level IR as
input.

lib/Conversion/MooreToCore/MooreToCore.cpp

@@ -1599,6 +1599,7 @@ static void populateLegality(ConversionTarget &target,
   target.addIllegalDialect<MooreDialect>();
   target.addLegalDialect<comb::CombDialect>();
   target.addLegalDialect<hw::HWDialect>();
+  target.addLegalDialect<seq::SeqDialect>();
   target.addLegalDialect<llhd::LLHDDialect>();
   target.addLegalDialect<ltl::LTLDialect>();
   target.addLegalDialect<mlir::BuiltinDialect>();