Support opt-in capability diagnostics for libraries

[bamarsha]

Currently, referencing a callable from a library will show warnings in addition to the primary error. The warnings are there so that you can understand why a callable is not supported and fix it if you have control of its source code. This doesn't work as well for the new diagnostics added by #1429, so they don't support this feature.

We would like to remove the functionality entirely for consistency and replace it with a feature that allows libraries to opt-in to diagnostics for specific targets (even though libraries are compiled as target-agnostic). This will also allow cleaning up some cruft from the new capability analyzer code.

[bettinaheim]

I believe this should be addressed with #1465 (now also merged to main). I'll close this - please reopen if you see something that isn't covered.

[bamarsha]

We still need to remove the old code for the diagnostics that should no longer be emitted, so I opened #1480 for that.