[FIRRTL] Use cached circuit symbol table for option group lookup.

Now that we've added a cached symbol table at the circuit level, we
can use that to look up option groups as well. This didn't have a
major performance improvement on some large designs, but in the
presence of more instance choices, this could help. Regardless, this
is one minor piece to clean up for #6772.

lib/Dialect/FIRRTL/Import/FIRParser.cpp

@@ -3883,10 +3883,7 @@ ParseResult FIRStmtParser::parseInstanceChoice() {
   SmallVector<PortInfo> modulePorts = defaultModule.getPorts();
 
   // Find the option group.
-  auto circuit =
-      builder.getBlock()->getParentOp()->getParentOfType<CircuitOp>();
-  auto optionGroup =
-      dyn_cast_or_null<OptionOp>(circuit.lookupSymbol(optionGroupName));
+  auto optionGroup = circuitSymTbl.lookup<OptionOp>(optionGroupName);
   if (!optionGroup)
     return emitError(loc,
                      "use of undefined option group '" + optionGroupName + "'");