DebugInfo: for loop backedge should be attributed to the start of the for statement.

Similar to r215768 (which fixed the same case for while loops). To quote
r215768's commit message:

"A little test case simplification - this could be simplified further,
though there are certainly interesting connections to the if/else
construct so I'm hesitant to remove that entirely though it does appear
somewhat unrelated.

(similar fix to r215766, related to PR19864)"

llvm-svn: 216297