c++: Quash -Wdangling-reference for member operator* [PR107488]

-Wdangling-reference complains here:

  std::vector<int> v = ...;
  std::vector<int>::const_iterator it = v.begin();
  while (it != v.end()) {
    const int &r = *it++; // warning
  }

because it sees a call to
__gnu_cxx::__normal_iterator<const int*, std::vector<int> >::operator*
which returns a reference and its argument is a TARGET_EXPR representing
the result of
__gnu_cxx::__normal_iterator<const int*, std::vector<int> >::operator++
But 'r' above refers to one of the int elements of the vector 'v', not
to a temporary object.  Therefore the warning is a false positive.

I suppose code like the above is relatively common (the warning broke
cppunit-1.15.1 and a few other projects), so presumably it makes sense
to suppress the warning when it comes to member operator*.  In this case
it's defined as

      reference
      operator*() const _GLIBCXX_NOEXCEPT
      { return *_M_current; }

and I'm guessing a lot of member operator* are like that, at least when
it comes to iterators.  I've looked at _Fwd_list_iterator,
_Fwd_list_const_iterator, __shared_ptr_access, _Deque_iterator,
istream_iterator, etc, and they're all like that, so adding #pragmas
would be quite tedious.  :/

	PR c++/107488

gcc/cp/ChangeLog:

	* call.cc (do_warn_dangling_reference): Quash -Wdangling-reference
	for member operator*.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wdangling-reference5.C: New test.

(cherry picked from commit 32a06ce38a38bf37db468f0e6c83520fcc221534)

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