Revert "[llvm] Make Sequence reverse-iterable"

This reverts commit e772216e
(and fixup 7f6c878a).

The build is broken with gcc5 host compiler:

In file included from
                 from mlir/lib/Dialect/Utils/StructuredOpsUtils.cpp:9:
tools/mlir/include/mlir/IR/BuiltinAttributes.h.inc:424:57: error: type/value mismatch at argument 1 in template parameter list for 'template<class ItTy, class FuncTy, class FuncReturnTy> class llvm::mapped_iterator'
                               std::function<T(ptrdiff_t)>>;
                                                         ^
tools/mlir/include/mlir/IR/BuiltinAttributes.h.inc:424:57: note:   expected a type, got 'decltype (seq<ptrdiff_t>(0, 0))::const_iterator'