cpp11-migrate: collect performance timers per source file

Performance timers captured in each transform for all files they process are now collected and arranged per source file in preparation for writing to disk.

This revision is the last piece of the initial implementation of performance timer capturing.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@183274 91177308-0d34-0410-b5e6-96231b3b80d8