Add compile flag for lost debug variables statistics

[Snowy1803]

Add a new -sil-stats-lost-variables flag to record statistics about lost debug variables. In combination with -sil-stats-output-file, it is possible to get a log file with the lost debug variables for each pass and function.

Using the included utils/process-stats-lost-variables, it is possible to get the number of lost variables for each pass, on a complete project.

It is possible to run it with the source compatibility test suite:

swift-source-compat-suite/runner.py --swift-branch main --projects swift-source-compat-suite/projects.json --include-repos 'path == "PROJECT_NAME"' --include-actions 'action.startswith("Build")' --swiftc path/to/swiftc --add-swift-flags '-Xllvm -sil-stats-lost-variables -Xllvm -sil-stats-output-file=/tmp/lostvariables.log'
swift/utils/process-stats-lost-variables /tmp/lostvariables.log

The compiler creates a CSV with the following format:

function, lostvars, [Stage Name], [Pass Name], [Pass Number], [Lost variables count], [Pass duration], [Mangled function name]

The process-stats-lost-variables creates a simpler CSV summarizing it:

[Pass name], [Lost variables]

The process-stats-lost-variables script also adds a header so it can easily be imported into Numbers.

[adrian-prantl]

This is great and going to be very useful! Can you add a paragraph to the debug info howto and the how to debug the compiler document?

[Snowy1803]

@swift-ci please test