[ADT] Remove illegal comparison of singular iterators from SmallSetTest

[ADT] Remove illegal comparison of singular iterators from SmallSetTest

This removes the assertion that a copy of a moved-from SmallSetIterator
equals the original, which is illegal due to SmallSetIterator including
an instance of a standard `std::set` iterator.

C++ [iterator.requirements.general] states that comparing singular
iterators has undefined result:

> Iterators can also have singular values that are not associated with
> any sequence. [...] Results of most expressions are undefined for
> singular values; the only exceptions are destroying an iterator that
> holds a singular value, the assignment of a non-singular value to an
> iterator that holds a singular value, and, for iterators that satisfy
> the Cpp17DefaultConstructible requirements, using a value-initialized
> iterator as the source of a copy or move operation.

This assertion triggers the following error in the GNU C++ Library in
debug mode under EXPENSIVE_CHECKS:

  /usr/include/c++/8.2.1/debug/safe_iterator.h:518:
  Error: attempt to compare a singular iterator to a singular iterator.

  Objects involved in the operation:
      iterator "lhs" @ 0x0x7fff86420670 {
        state = singular;
      }
      iterator "rhs" @ 0x0x7fff86420640 {
        state = singular;
      }

Patch by Eugene Sharygin.

Reviewers: fhahn, dblaikie, chandlerc

Reviewed By: fhahn, dblaikie

Differential Revision: https://reviews.llvm.org/D53793

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@345712 91177308-0d34-0410-b5e6-96231b3b80d8