lld: use `std::make_unique` rather than spelling out types twice

[compnerd]

Summary

The LLVM code base already uses C++14, use std::make_unique
to avoid the explicit constructor invocation via new and to avoid
spelling out the type twice.

[int3]

Seems fine but this code is part of the old lld-macho and will likely be removed in the near future

[compnerd]

rG9cb2badc596497f49ec2631bb5b8a2300656e75c

lld/lib/ReaderWriter/MachO/MachONormalizedFileToAtoms.cpp

 llvm::Expected<std::unique_ptr<lld::File>>
 objectToAtoms(const NormalizedFile &normalizedFile, StringRef path,
               bool copyRefs) {
-  std::unique_ptr<MachOFile> file(new MachOFile(path));
+  auto file = std::make_unique<MachOFile>(path);
   if (auto ec = normalizedObjectToAtoms(file.get(), normalizedFile, copyRefs))
     return std::move(ec);
   return std::unique_ptr<File>(std::move(file));

 dylibToAtoms(const NormalizedFile &normalizedFile, StringRef path,
              bool copyRefs) {
   // Instantiate SharedLibraryFile object.
-  std::unique_ptr<MachODylibFile> file(new MachODylibFile(path));
+  auto file = std::make_unique<MachODylibFile>(path);
   if (auto ec = normalizedDylibToAtoms(file.get(), normalizedFile, copyRefs))
     return std::move(ec);
   return std::unique_ptr<File>(std::move(file));