gdbsupport: mark array_view::slice with [[nodiscard]]

gdbsupport: mark array_view::slice with [[nodiscard]]

I (almost) had a bug where I did:

    buffer.slice (...)

but I meant:

    buffer = buffer.slice (...)

The first one does nothing, it creates a new array_view but without
using it, it's useless.  Mark the slice methods with [[nodiscard]]
(which is standard C++17) so that error would generate a warning.

I guess that many functions could be marked as nodiscard, essentially
function that is pure (doesn't have side-effects).  But this one seems
particularly easy to mis-use.

Change-Id: Ib39a0a65a5728a3cfd68a02ae31635810baeaccb
Approved-By: Tom Tromey <tom@tromey.com>