Add TaskMap for iterating a function over a set of integers

Summary:
Many parallel tasks just want to iterate over all the possible numbers from 0 to N-1.  Rather than enqueue N work items, instead just "map" the function across the requested integer space.

Reviewers: clayborg, labath, tberghammer, zturner

Reviewed By: clayborg, zturner

Subscribers: zturner, lldb-commits

Differential Revision: https://reviews.llvm.org/D32757
Patch by Scott Smith <scott.smith@purestorage.com>.

llvm-svn: 302223