Consider the following Ada code:

       procedure Nested (L, U : Integer) is
          subtype Small_Type is Integer range L .. U;
          type Record_Type (I : Small_Type := L) is record
             S : String (1 .. I);
          end record;
          type Array_Type is array (Integer range <>) of Record_Type;

          A1 : Array_Type :=
            (1 => (I => 0, S => <>),
             2 => (I => 1, S => "A"),
             3 => (I => 2, S => "AB"));

          procedure Discard (R : Record_Type) is
          begin
             null;
          end Discard;

       begin
          Discard (A1 (1));  -- STOP
       end;

Trying to print a slice of that array currently yields:

    (gdb) p a1(1..3)
    $1 = ((i => 0, s => ""), (i => 0, s => ""), (i => 0, s => ""))

We expected instead:

    (gdb) p a1(1..3)
    $1 = ((i => 0, s => ""), (i => 1, s => "A"), (i => 2, s => "AB"))

This is because the functions we use in ada-lang.c to create the type
of the array slice (ada_value_slice and ada_value_slice_from_ptr) was
not taking into account the stride of the array. This patch fixes this.

gdb/ChangeLog:

        * ada-lang.c (ada_value_slice_from_ptr): Take array stride into
        account when creating the array type of the slice.
        (ada_value_slice): Likewise.

gdb/testsuite/ChangeLog:

        * gdb.ada/dyn_stride.exp: Add slice test.

Note that, with the current use of ada_value_slice, the enhancement
to handle dynamic array strides seems unnecessary, because I do not
see how an array with a dynamic stride can be referenced by either
by reference or pointer. Since references are coerced to array pointers,
in both cases, the slice is performed by ada_value_slice_from_ptr.
But ada_value_slice is enhanced nonetheless, in the spirit of making
the code more robust, in case we missed something, and also as similar
as possible with its from_ptr counterpart.

tested on x86_64-linux.