c++: avoid -Wdangling-reference for std::span-like classes [PR110358]

c++: avoid -Wdangling-reference for std::span-like classes [PR110358]

Real-world experience shows that -Wdangling-reference triggers for
user-defined std::span-like classes a lot.  We can easily avoid that
by considering classes like

    template<typename T>
    struct Span {
      T* data_;
      std::size len_;
    };

to be std::span-like, and not warning for them.  Unlike the previous
patch, this one considers a non-union class template that has a pointer
data member and a trivial destructor as std::span-like.

	PR c++/110358
	PR c++/109640

gcc/cp/ChangeLog:

	* call.cc (reference_like_class_p): Don't warn for std::span-like
	classes.

gcc/ChangeLog:

	* doc/invoke.texi: Update -Wdangling-reference description.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wdangling-reference18.C: New test.
	* g++.dg/warn/Wdangling-reference19.C: New test.
	* g++.dg/warn/Wdangling-reference20.C: New test.