gdbsupport: make filtered_iterator work with pointers

gdbsupport: make filtered_iterator work with pointers

It's currently not possible to use filtered_iterator with a pointer as
the base iterator type.  This patch makes it possible.  The indended
usage is:

  Foo array[12];
  Foo *begin = array;
  Foo *end = array + ARRAY_SIZE (array);
  filtered_iterator<Foo *, FooFilter> (begin, end);

Here are the things that needed changing:

 - Give filtered_iterator a constructor where the caller provides
   already constructed begin and end iterators.  filtered_iterator
   currently assumes that default-constructing a BaseIterator will
   produce a valid "end" iterator.  This is not the case if BaseIterator
   is a pointer.  The caller needs to pass in the end of the array /
   region to iterate on as the end.

 - Typedefs of member types like wouldn't work:

     typedef typename BaseIterator::value_type value_type;

   The compiler would complain that it's not possible to apply `::` to
   type `BaseIterator` (aka `Foo *`).  Use std::iterator_traits to fix
   it [1].

 - Similarly, the compiler would complain about the use of
   `BaseIterator::operator*` in the return type of
   `filtered_iterator::operator*`.  Fix this by using `decltype(auto)`
   as the return type.  This lets the compiler deduce the return type
   from the return statement.  Unlike `auto`, `decltype(auto)` perfectly
   preserves the "cvref-ness" of the deduced return type.  If the return
   expression yields a `Foo &`, then the function will return a `Foo &`
   (which is what we want), whereas it would return a `Foo` if we used
   just `auto`.

Improve the filtered_iterator unit tests to run the same tests but with
pointers as iterators.  Because the filtered_iterator objects are
initialized differently in the two scenarios, I chose to copy the
existing code and adapt it.  It would probably be possible to add a
layer of abstraction to avoid code duplication, but it would end up more
complicated and messy.  If we ever add a third scenario, we can revisit
that.

[1] https://en.cppreference.com/w/cpp/iterator/iterator_traits.html

Change-Id: Id962ffbcd960a705a82bc5eb4808b4fe118a2761
Approved-By: Tom Tromey <tom@tromey.com>