[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/Object.h b/llvm/tools/llvm-objcopy/MachO/Object.h
index 703c31f..a2c0074 100644
--- a/llvm/tools/llvm-objcopy/MachO/Object.h
+++ b/llvm/tools/llvm-objcopy/MachO/Object.h
@@ -165,7 +165,7 @@
   // The referenced symbol entry. Set if !Scattered && Extern.
   Optional<const SymbolEntry *> Symbol;
   // The referenced section. Set if !Scattered && !Extern.
-  Optional<const Section *> Section;
+  Optional<const Section *> Sec;
   // True if Info is a scattered_relocation_info.
   bool Scattered;
   // True if the r_symbolnum points to a section number (i.e. r_extern=0).