gdbsupport: remove variadicity from basic_safe_iterator constructors

gdbsupport: remove variadicity from basic_safe_iterator constructors

Change the constructors to accept `Iterator` objects directly.  This
requires the callers to explicitly pass `Iterator` object (unless
perhaps `Iterator` has a non-explicit one-argument constructor.

The rationale is the same as the previous patch: make the code easier to
follow and make it easier to fix build errors, at the expense of making
callers more explicit.

Change-Id: Icd2a4ef971456ca250f96227a9b83c935d619451
Approved-By: Tom Tromey <tom@tromey.com>