Improve gdb::array_view ctor from contiguous containers

While reading the interface of gdb::array_view, I realized that the
constructor that builds an array_view on top of a contiguous container
(such as std::vector, std::array or even gdb::array_view) can be
missused.

Lets consider the following code sample:

	struct Parent
	{
	  Parent (int a): a { a } {}
	  int a;
	};

	std::ostream &operator<< (std::ostream& os, const Parent & p)
	{ os << "Parent {a=" << p.a << "}"; return os; }

	struct Child : public Parent
	{
	  Child (int a, int b): Parent { a }, b { b } {}
	  int b;
	};

	std::ostream &operator<< (std::ostream& os, const Child & p)
	{ os << "Child {a=" << p.a << ", b=" << p.b << "}"; return os; }

	template <typename T>
	void print (const gdb::array_view<const T> &p)
	{
	  std::for_each (p.begin (), p.end (), [](const T &p) { std::cout << p << '\n'; });
	}

Then with the current interface nothinng prevents this usage of
array_view to be done:

	const std::array<Child, 3> elts = {
	  Child {1, 2},
	  Child {3, 4},
	  Child {5, 6}
	};
	print_all<Parent> (elts);

This compiles fine and produces the following output:

	Parent {a=1}
	Parent {a=2}
	Parent {a=3}

which is obviously wrong.  There is nowhere in memory a Parent-like
object for which the A member is 2 and this call to print_all<Parent>
shold not compile at all (calling print_all<Child> is however fine).

This comes down to the fact that a Child* is convertible into a Parent*,
and that an array view is constructed to a pointer to the first element
and a size.  The valid type pointed to that can be used with this
constructor are restricted using SFINAE, which requires that a
pointer to a member into the underlying container can be converted into a
pointer the array_view's data type.

This patch proposes to change the constraints on the gdb::array_view
ctor which accepts a container now requires that the (decayed) type of
the elements in the container match the (decayed) type of the array_view
being constructed.

Applying this change required minimum adjustment in GDB codebase, which
are also included in this patch.

Tested by rebuilding.

gdbsupport/array-view.h

@@ -139,9 +139,10 @@ public:
   template<typename Container,
 	   typename = Requires<gdb::Not<IsDecayedT<Container>>>,
 	   typename
-	     = Requires<std::is_convertible
-			<decltype (std::declval<Container> ().data ()),
-			 T *>>,
+	     = Requires<DecayedConvertible
+			<typename std::remove_pointer
+			 <decltype (std::declval<Container> ().data ())
+			 >::type>>,
 	   typename
 	     = Requires<std::is_convertible
 			<decltype (std::declval<Container> ().size ()),