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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337787 91177308-0d34-0410-b5e6-96231b3b80d8
