[llvm-objcopy][MachO] Fix build

Some compilers are confused when the same name is used in different contexts.
Rename the field Section to unbreak the build.
(Caught by the buildbot http://lab.llvm.org:8011/builders/clang-with-thin-lto-ubuntu/builds/22374)
diff --git a/llvm/tools/llvm-objcopy/MachO/MachOWriter.cpp b/llvm/tools/llvm-objcopy/MachO/MachOWriter.cpp
index 4a8d1ae..1cb67d0 100644
--- a/llvm/tools/llvm-objcopy/MachO/MachOWriter.cpp
+++ b/llvm/tools/llvm-objcopy/MachO/MachOWriter.cpp
@@ -243,7 +243,7 @@
         if (!RelocInfo.Scattered) {
           const uint32_t SymbolNum = RelocInfo.Extern
                                          ? (*RelocInfo.Symbol)->Index
-                                         : (*RelocInfo.Section)->Index;
+                                         : (*RelocInfo.Sec)->Index;
           RelocInfo.setPlainRelocationSymbolNum(SymbolNum, IsLittleEndian);
         }
         if (IsLittleEndian != sys::IsLittleEndianHost)