clang-modernize: Reset LoopConvert's TU tracking info per TU

The LoopConvert transform makes use of data structures it builds up over
the course of transforming a TU. Until now, these data structures
weren't being cleared out before the next TU was being processed.

Fixes PR17253.

Differential Revision: http://llvm-reviews.chandlerc.com/D175

llvm-svn: 191448