Add target to ApiCompat.proj that determines when the baseline shrinks

[wtgodbe]

Right now, unless a dev manually updates the baseline files for APICompat, we have no way of knowing if a check that's currently in the baseline has stopped triggering. We get an error when new checks pop up, so we'll know if the baseline grows, but nothing happens when old checks disappear, so we don't know if the baseline shrinks.

We could try writing to the baseline during every build, in addition to throwing an error if we trigger checks that weren't previously in the baseline.

CC @ericstj @terrajobst

[ericstj]

Generically, I believe that is by-design. The baseline files often are shared per framework and may contain entries only needed for one of many frameworks. This could be changed but would be a feature in APICompat in dotnet/arcade.

That said, you could hack something together that captures the output of the tool when run and compares that to the checked in baseline. Here's some psuedo code:

run apicompat -baseline original.txt
run apicompat > latest.txt
readlines original.txt -> @(orig)
readlines latest.txt > @(latest)
@(orig) exclude @(latest) -> @(orphaned)
Error @(orphaned)

[ViktorHofer]

Closing as I recently leveraged the ApiCompat's ValidateBaseline switch in ApiCompat.proj. That said, it's disabled right now because of a temporary issue tracked in #68730.