gdbsupport: use iterator range in parallel_for_each interface

I think it would be convenient for parallel_for_each to pass an
iterator_range to the worker function, instead of separate begin and end
parameters.  This allows using a ranged for loop directly.

Change-Id: I8f9681da65b0eb00b738379dfd2f4dc6fb1ee612
Approved-By: Tom Tromey <tom@tromey.com>