RequirementMachine: Add concrete nesting depth check

Configured with -requirement-machine-max-concrete-nesting= frontend flag.

lib/Frontend/CompilerInvocation.cpp

@@ -950,6 +950,17 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     }
   }
 
+  if (const Arg *A = Args.getLastArg(OPT_requirement_machine_max_concrete_nesting)) {
+    unsigned limit;
+    if (StringRef(A->getValue()).getAsInteger(10, limit)) {
+      Diags.diagnose(SourceLoc(), diag::error_invalid_arg_value,
+                     A->getAsString(Args), A->getValue());
+      HadError = true;
+    } else {
+      Opts.RequirementMachineMaxConcreteNesting = limit;
+    }
+  }
+
   return HadError || UnsupportedOS || UnsupportedArch;
 }