Add frontend flag -enable-new-llvm-pass-manager to enable new llvm pass manager

Author: aschwaighofer

include/swift/Option/FrontendOptions.td

@@ -1072,4 +1072,8 @@ def concurrency_model :
 def concurrency_model_EQ :
     Joined<["-"], "concurrency-model=">,
     Alias<concurrency_model>;
+
+def enable_new_llvm_pass_manager :
+  Flag<["-"], "enable-new-llvm-pass-manager">,
+  HelpText<"Enable new llvm pass manager">;
 } // end let Flags = [FrontendOption, NoDriverOption, HelpHidden]

lib/Frontend/CompilerInvocation.cpp

@@ -2365,6 +2365,9 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
       Opts.SwiftAsyncFramePointer = SwiftAsyncFramePointerKind::Never;
   }
 
+  if (Args.hasArg(OPT_enable_new_llvm_pass_manager))
+    Opts.LegacyPassManager = false;
+
   return false;
 }