[STLExtras] Use return type from operator* of the wrapped iter.

Currently make_early_inc_range cannot be used with iterators with
operator* implementations that do not return a reference.

Most notably in the LLVM codebase, this means the User iterator ranges
cannot be used with make_early_inc_range, which slightly simplifies
iterating over ranges while elements are removed.

Instead of directly using BaseT::reference as return type of operator*,
this patch uses decltype to get the actual return type of the operator*
implementation in WrappedIteratorT.

This patch also updates a few places to use make use of
make_early_inc_range.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D93992