[Debug] Annotate compiler generated range-for loop variables.

Summary:
This change aims to simplify debugging by annotating the range-for loop artificial variables (range, begin, end) with the scope depth. 


Reviewers: rsmith, dblaikie

Reviewed By: dblaikie

Subscribers: dblaikie, cfe-commits

Tags: #debug-info

Differential Revision: https://reviews.llvm.org/D42813

llvm-svn: 325175