[ThinLTO] Handle -emit-llvm* in ThinLTO backends

[tejohnson]

Summary

Use PreCodeGenModuleHook to invoke the correct writer when emitting LLVM
IR, returning false to skip codegen from within thinBackend.

[pcc]

Why not call Module::print() or WriteBitcodeToFile directly instead of creating a pass manager?

[tejohnson]

Implement review suggestions

[pcc]

LGTM

lib/CodeGen/BackendUtil.cpp

auto AddStream = [&](size_t Task) {
     return llvm::make_unique<lto::NativeObjectStream>(std::move(OS));
   };
   lto::Config Conf;
   Conf.CPU = TOpts.CPU;
   Conf.CodeModel = getCodeModel(CGOpts);
   Conf.MAttrs = TOpts.Features;
   Conf.RelocModel = getRelocModel(CGOpts);
   Conf.CGOptLevel = getCGOptLevel(CGOpts);
-  Conf.CGFileType = getCodeGenFileType(Action);
   initTargetOptions(Conf.Options, CGOpts, TOpts, LOpts, HeaderOpts);
   Conf.SampleProfile = std::move(SampleProfile);
+  switch (Action) {
+  case Backend_EmitNothing:
+    Conf.PreCodeGenModuleHook = [](size_t Task, const Module &Mod) {
+      return false;
+    };
+    break;
+  case Backend_EmitLL:
+    Conf.PreCodeGenModuleHook = [&](size_t Task, const Module &Mod) {
+      M->print(*OS, nullptr, CGOpts.EmitLLVMUseLists);
+      return false;
+    };
+    break;
+  case Backend_EmitBC:
+    Conf.PreCodeGenModuleHook = [&](size_t Task, const Module &Mod) {
+      WriteBitcodeToFile(M, *OS, CGOpts.EmitLLVMUseLists);
+      return false;
+    };
+    break;
+  default:
+    Conf.CGFileType = getCodeGenFileType(Action);
+    break;
+  }
   if (Error E = thinBackend(
           Conf, 0, AddStream, *M, *CombinedIndex, ImportList,
           ModuleToDefinedGVSummaries[M->getModuleIdentifier()], ModuleMap)) {
     handleAllErrors(std::move(E), [&](ErrorInfoBase &EIB) {
       errs() << "Error running ThinLTO backend: " << EIB.message() << '\n';
     });
   }
 }

test/CodeGen/thinlto-emit-llvm.c

+// Test to ensure -emit-llvm and -emit-llvm-bc work when invoking the
+// ThinLTO backend path.
+// RUN: %clang -O2 %s -flto=thin -c -o %t.o
+// RUN: llvm-lto -thinlto -o %t %t.o
+// RUN: %clang_cc1 -O2 -x ir %t.o -fthinlto-index=%t.thinlto.bc -emit-llvm -o - | FileCheck %s
+// RUN: %clang_cc1 -O2 -x ir %t.o -fthinlto-index=%t.thinlto.bc -emit-llvm-bc -o - | llvm-dis -o - | FileCheck %s
+
+// CHECK: define void @foo()
+void foo() {
+}