[Debugify] Export per-pass debug info loss statistics

Add a -debugify-export option to opt. This exports per-pass `debugify`
loss statistics to a file in CSV format.

For some interesting numbers on debug value loss during an -O2 build
of the sqlite3 amalgamation, see the review thread.

Differential Revision: https://reviews.llvm.org/D49003

llvm-svn: 337787