[firtool] use FIRRTL layer syntax for specialization CL options

[youngar]

The syntax for nested layers doesn't match the FIRRTL spec and instead matches the MLIR representation. It would seem to make more sense to use the FIRRTL syntax.
Namely, firtool -enable-layers=Verification.Assert instead of firtool -enable-layers=Verification::Assert.

[youngar]

Fix would be to this code:

circt/lib/Dialect/FIRRTL/Import/FIRParser.cpp

Lines 5581 to 5593 in 3c12682

	// Helper to transform a layer name specification of the form `A::B::C` into
	// a SymbolRefAttr.
	auto parseLayerName = [&](StringRef name) {
	// Parse the layer name into a SymbolRefAttr.
	auto [head, rest] = name.split("::");
	SmallVector<FlatSymbolRefAttr> nestedRefs;
	while (!rest.empty()) {
	StringRef next;
	std::tie(next, rest) = rest.split("::");
	nestedRefs.push_back(FlatSymbolRefAttr::get(getContext(), next));
	}
	return SymbolRefAttr::get(getContext(), head, nestedRefs);
	};