IRGen: Add a frontend option to force single LLVM module emission in multithreaded mode

This allows to experiment with single module LLVM emission without
having to change drivers that expect multiple output files.

rdar://94744623

lib/Frontend/CompilerInvocation.cpp

@@ -2299,6 +2299,9 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
                      "-num-threads");
     }
   }
+  Opts.UseSingleModuleLLVMEmission =
+      Opts.NumThreads != 0 &&
+      Args.hasArg(OPT_enable_single_module_llvm_emission);
 
   if (SWIFT_ENABLE_GLOBAL_ISEL_ARM64 &&
       Triple.getArch() == llvm::Triple::aarch64 &&