[gdbsupport] Use std::span-style iterators for gdb::array_view

[gdbsupport] Use std::span-style iterators for gdb::array_view

There's a plan to replace gdb::array_view with std::span (PR31422), and making
gdb::array_view more like std::span helps with that.

One difference is that std::span has:
...
constexpr iterator begin() const noexcept;
constexpr const_iterator cbegin() const noexcept;
...
while gdb::array_view has:
...
constexpr T *begin () noexcept;
constexpr const T *begin () const noexcept;
...

Fix this by renaming the second variant to cbegin, and making the first
variant const.

Likewise for gdb::array_view::end.

Tested on aarch64-linux.

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