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-cfi-verify/lib/FileAnalysis.cpp b/llvm/tools/llvm-cfi-verify/lib/FileAnalysis.cpp
index 4bf308f..ff2ce46 100644
--- a/llvm/tools/llvm-cfi-verify/lib/FileAnalysis.cpp
+++ b/llvm/tools/llvm-cfi-verify/lib/FileAnalysis.cpp
@@ -453,11 +453,13 @@
     if (!Section.getName(SectionName) && SectionName == ".plt")
       continue;
 
-    Expected<StringRef> Contents = Section.getContents();
-    if (!Contents)
-      return Contents.takeError();
-    ArrayRef<uint8_t> SectionBytes = arrayRefFromStringRef(*Contents);
+    StringRef SectionContents;
+    if (Section.getContents(SectionContents))
+      return make_error<StringError>("Failed to retrieve section contents",
+                                     inconvertibleErrorCode());
 
+    ArrayRef<uint8_t> SectionBytes((const uint8_t *)SectionContents.data(),
+                                   Section.getSize());
     parseSectionContents(SectionBytes,
                          {Section.getAddress(), Section.getIndex()});
   }