Rewrite llvm-lto's codegen() using ThinCodeGenerator::run(). NFC-ish.

[aprantl]

Summary

This patch rewrites llvm-lto's codegen() using ThinCodeGenerator::run().
The change is NFC-ish (if you pass multiple files, the all files will now be codegen'd first and then written to disk instead of writing each module to disk after it has been codegen'd.

This is supposed to make https://reviews.llvm.org/D33151 simpler.

[mehdi_amini]

Thanks!

llvm/trunk/tools/llvm-lto/llvm-lto.cpp

void codegen() {
     if (InputFilenames.size() != 1 && !OutputFilename.empty())
       report_fatal_error("Can't handle a single output filename and multiple "
                          "input files, do not provide an output filename and "
                          "the output files will be suffixed from the input "
                          "ones.");
     if (!ThinLTOIndex.empty())
       errs() << "Warning: -thinlto-index ignored for codegen stage";
 
+    std::vector<std::unique_ptr<MemoryBuffer>> InputBuffers;
     for (auto &Filename : InputFilenames) {
       LLVMContext Ctx;
-      auto TheModule = loadModule(Filename, Ctx);
-
-      auto Buffer = ThinGenerator.codegen(*TheModule);
-      std::string OutputName = OutputFilename;
-      if (OutputName.empty()) {
-        OutputName = Filename + ".thinlto.o";
-      }
-      if (OutputName == "-") {
-        outs() << Buffer->getBuffer();
+      auto InputOrErr = MemoryBuffer::getFile(Filename);
+      error(InputOrErr, "error " + CurrentActivity);
+      InputBuffers.push_back(std::move(*InputOrErr));
+      ThinGenerator.addModule(Filename, InputBuffers.back()->getBuffer());
+    ThinGenerator.setCodeGenOnly(true);
+    ThinGenerator.run();
+    for (auto BinName :
+         zip(ThinGenerator.getProducedBinaries(), InputFilenames)) {
+      std::string OutputName = OutputFilename;
+      if (OutputName.empty())
+        OutputName = std::get<1>(BinName) + ".thinlto.o";
+      else if (OutputName == "-") {
+        outs() << std::get<0>(BinName)->getBuffer();
         return;
       }
 
       std::error_code EC;
       raw_fd_ostream OS(OutputName, EC, sys::fs::OpenFlags::F_None);
       error(EC, "error opening the file '" + OutputName + "'");
-      OS << Buffer->getBuffer();
+      OS << std::get<0>(BinName)->getBuffer();
     }
   }
 
   /// Full ThinLTO process
   void runAll() {
     if (!OutputFilename.empty())
       report_fatal_error("Do not provide an output filename for ThinLTO "
                          " processing, the output files will be suffixed from "