[llvm-lipo] Add missing cast

Add missing cast (to correctly sum 32-bit integers).

Test plan: make check-all


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@374945 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/tools/llvm-lipo/llvm-lipo.cpp b/tools/llvm-lipo/llvm-lipo.cpp
index ba413b1..a063fce 100644
--- a/tools/llvm-lipo/llvm-lipo.cpp
+++ b/tools/llvm-lipo/llvm-lipo.cpp
@@ -688,7 +688,8 @@
     return sys::fs::can_execute(S.getBinary()->getFileName());
   });
   const uint64_t OutputFileSize =
-      FatArchList.back().offset + FatArchList.back().size;
+      static_cast<uint64_t>(FatArchList.back().offset) +
+      FatArchList.back().size;
   Expected<std::unique_ptr<FileOutputBuffer>> OutFileOrError =
       FileOutputBuffer::create(OutputFileName, OutputFileSize,
                                IsExecutable ? FileOutputBuffer::F_executable