Introduce basic_safe_range

Introduce basic_safe_range

This introduces the basic_safe_range class, which can be used to
create a basic_safe_iterator.  This also changes basic_safe_iterator
in two ways.

First, it simplifies the constructor.  This seemed unnecessarily
complicated to me, and keeping it this way would prevent the second
change...

... which is to add a second constructor for initializing the
one-past-the-end iterator that is stored in basic_safe_iterator.

gdb/ChangeLog
2019-12-12  Tom Tromey  <tom@tromey.com>

	* gdbsupport/safe-iterator.h (basic_safe_iterator): Simplify.  Add
	second constructor.
	(basic_safe_range): New class.

Change-Id: Ib351ef6fd435129a5053c64e5561877e1459ab37