[clang] Fix build after 537344f

This fixes the following build failure on lldb-x86_64-debian:

/llvm-project/clang/lib/Serialization/ASTReader.cpp:1453:12: error: call to deleted constructor of 'llvm::Error'
    return Err;
           ^~~
/llvm-project/llvm/include/llvm/Support/Error.h:189:3: note: 'Error' has been explicitly marked deleted here
  Error(const Error &Other) = delete;
  ^
/llvm-project/llvm/include/llvm/Support/Error.h:496:18: note: passing argument to parameter 'Err' here
  Expected(Error Err)
                 ^

clang/lib/Serialization/ASTReader.cpp

@@ -1450,7 +1450,7 @@ ASTReader::readSLocOffset(ModuleFile *F, unsigned Index) {
   SavedStreamPosition SavedPosition(Cursor);
   if (llvm::Error Err = Cursor.JumpToBit(F->SLocEntryOffsetsBase +
                                          F->SLocEntryOffsets[Index]))
-    return Err;
+    return std::move(Err);
 
   Expected<llvm::BitstreamEntry> MaybeEntry = Cursor.advance();
   if (!MaybeEntry)