[LTO][NPM] Default to using NPM under ENABLE_EXPERIMENTAL_NEW_PASS_MANAGER

[aeubanks]

Summary

This affects users of LTO that don't explicitly set UseNewPM.

[MaskRay]

This will not give users a way to switch back to the legacy pass manager. We should add a linker option first (D92916), then teach ToolChains/CommonArgs.cpp to pass the option, finally we can do this. So please hold off on this a bit :)

[aeubanks]

Good now?

[MaskRay]

Yes!

llvm/include/llvm/LTO/Config.h

 // configure LTO.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_LTO_CONFIG_H
 #define LLVM_LTO_CONFIG_H
 
 #include "llvm/ADT/DenseSet.h"
+#include "llvm/Config/llvm-config.h"
 #include "llvm/IR/DiagnosticInfo.h"
 #include "llvm/IR/GlobalValue.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/Passes/PassBuilder.h"
 #include "llvm/Support/CodeGen.h"
 #include "llvm/Target/TargetOptions.h"
 
 #include <functional>
struct Config {
   Optional<Reloc::Model> RelocModel = Reloc::PIC_;
   Optional<CodeModel::Model> CodeModel = None;
   CodeGenOpt::Level CGOptLevel = CodeGenOpt::Default;
   CodeGenFileType CGFileType = CGFT_ObjectFile;
   unsigned OptLevel = 2;
   bool DisableVerify = false;
 
   /// Use the new pass manager
-  bool UseNewPM = false;
+  bool UseNewPM = LLVM_ENABLE_NEW_PASS_MANAGER;
 
   /// Flag to indicate that the optimizer should not assume builtins are present
   /// on the target.
   bool Freestanding = false;
 
   /// Disable entirely the optimizer, including importing for ThinLTO
   bool CodeGenOnly = false;