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/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp b/llvm/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp
index f99868d..2fd677a 100644
--- a/llvm/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp
+++ b/llvm/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp
@@ -792,10 +792,8 @@
   if (!IsVirtual && !IsZeroInit) {
     // In either case, set the location of the unrelocated section in memory,
     // since we still process relocations for it even if we're not applying them.
-    if (Expected<StringRef> E = Section.getContents())
-      data = *E;
-    else
-      return E.takeError();
+    if (auto EC = Section.getContents(data))
+      return errorCodeToError(EC);
     pData = data.data();
   }