[lanai] Add back the ability to set the memory model with --mcmodel.

Sometimes between April 2023 and today the option disappeared. I wasn't able to trace when it went away through the different refactoring.

Author: petergrind3

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -3055,6 +3055,8 @@ void tools::addMCModel(const Driver &D, const llvm::opt::ArgList &Args,
       else if (CM == "medany")
         CM = "large";
       Ok = CM == "small" || CM == "medium" || CM == "large";
+    } else if (Triple.getArch() == llvm::Triple::lanai) {
+      Ok = llvm::is_contained({"small", "medium", "large"}, CM);
     }
     if (Ok) {
       CmdArgs.push_back(Args.MakeArgString("-mcmodel=" + CM));