Change the as_*_interp functions to use dynamic_cast

Change the as_*_interp functions to use dynamic_cast

This changes the various as_*_interp functions to be implemented using
dynamic_cast.  I believe this is a small improvement, because it is
more typesafe -- the C++ runtime does the type-checking for us.

ChangeLog
2018-05-25  Tom Tromey  <tom@tromey.com>

	* tui/tui-interp.c (as_tui_interp): Use dynamic_cast.
	* mi/mi-interp.c (as_mi_interp): Use dynamic_cast.
	* cli/cli-interp.c (as_cli_interp): Use dynamic_cast.