Revert r360876 "[Object] Change object::SectionRef::getContents() to return Expected<StringRef>"

It broke the Clang build, see llvm-commits thread.

> Expected<ArrayRef<uint8_t>> may be better but use Expected<StringRef> for now.
>
> Follow-up of D61781.

llvm-svn: 360878
diff --git a/llvm/tools/llvm-readobj/MachODumper.cpp b/llvm/tools/llvm-readobj/MachODumper.cpp
index 5149f46..5e82f1c 100644
--- a/llvm/tools/llvm-readobj/MachODumper.cpp
+++ b/llvm/tools/llvm-readobj/MachODumper.cpp
@@ -483,8 +483,15 @@
       }
     }
 
-    if (opts::SectionData && !Section.isBSS())
-      W.printBinaryBlock("SectionData", unwrapOrError(Section.getContents()));
+    if (opts::SectionData) {
+      bool IsBSS = Section.isBSS();
+      if (!IsBSS) {
+        StringRef Data;
+        error(Section.getContents(Data));
+
+        W.printBinaryBlock("SectionData", Data);
+      }
+    }
   }
 }
 
@@ -653,7 +660,8 @@
   if (StackMapSection == object::SectionRef())
     return;
 
-  StringRef StackMapContents = unwrapOrError(StackMapSection.getContents());
+  StringRef StackMapContents;
+  StackMapSection.getContents(StackMapContents);
   ArrayRef<uint8_t> StackMapContentsArray =
       arrayRefFromStringRef(StackMapContents);