[llvm-objcopy][MachO] Recompute and update offset/size fields in the writer

Summary:
Recompute and update offset/size fields so that we can implement llvm-objcopy options like --only-section.

This patch is the first step and focuses on supporting load commands that covered by existing tests: executable files and
dynamic libraries are not supported.

Reviewers: alexshap, rupprecht, jhenderson

Reviewed By: alexshap, rupprecht

Subscribers: compnerd, jakehehrlich, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62652

llvm-svn: 362863
diff --git a/llvm/tools/llvm-objcopy/MachO/MachOObjcopy.cpp b/llvm/tools/llvm-objcopy/MachO/MachOObjcopy.cpp
index 80fb185..19343b6 100644
--- a/llvm/tools/llvm-objcopy/MachO/MachOObjcopy.cpp
+++ b/llvm/tools/llvm-objcopy/MachO/MachOObjcopy.cpp
@@ -58,6 +58,8 @@
     return createFileError(Config.InputFilename, std::move(E));
 
   MachOWriter Writer(*O, In.is64Bit(), In.isLittleEndian(), Out);
+  if (auto E = Writer.finalize())
+    return E;
   return Writer.write();
 }