gdbsupport: add checked_static_cast

This commit was inspired by these mailing list posts:

  https://sourceware.org/pipermail/gdb-patches/2022-June/190323.html
  https://sourceware.org/pipermail/gdb-patches/2022-April/188098.html



The idea is to add a new function gdb::checked_static_cast, which can,
in some cases, be used as a drop-in replacement for static_cast.  And
so, if I previously wrote this:

  BaseClass *base = get_base_class_pointer ();
  DerivedClass *derived = static_cast<DerivedClass *> (base);

I can now write:

  BaseClass *base = get_base_class_pointer ();
  DerivedClass *derived = gdb::checked_static_cast<DerivedClass *> (base);

The requirement is that BaseClass and DerivedClass must be
polymorphic.

The benefit of making this change is that, when GDB is built in
developer mode, a run-time check will be made to ensure that `base`
really is of type DerivedClass before the cast is performed.  If
`base` is not of type DerivedClass then GDB will assert.

In a non-developer build gdb::checked_static_cast is equivalent to a
static_cast, and there should be no performance difference.

This commit adds the support function, but does not make use of this
function, a use will be added in the next commit.

Co-Authored-By: Pedro Alves <pedro@palves.net>
Co-Authored-By: Tom Tromey <tom@tromey.com>