[cas] Fix caching of diagnostics using getCustomDiagID

[benlangmuir]

Custom diagnostics need to be serialized so that they can be registered with the diagnostic engine during replay. If any custom diagnostics are emitted, we capture all the known custom diagnostics. While this could theoretically be wasteful, in practice custom diagnostics are created only when they will be emitted. Note: this also fixes clang/test/CAS/analyze-action.c.

[benlangmuir]

@swift-ci please test llvm

[cachemeifyoucan]

Does it work if you only capture the customized diagnostics being emitted if that hasn't been registered before? I think DiagID doesn't have to match between two diagnostics engine? I could miss something or it might not work during the implementation.

[benlangmuir]

Does it work if you only capture the customized diagnostics being emitted if that hasn't been registered before?

In theory yes, but then we need to keep a mapping between the DiagID in the original engine and whatever we're going to use in the serializer. Registration of custom diagnostics always uses the next available DiagID, so we can't just create an arbitrary mapping, and it needs to be consistent if the same diagnostic is emitted a second time. That seems like an unnecessary complication, which is why I didn't do it. If we find cases where this is a real problem then we can revisit, but in practice custom diagnostics are almost always registered immediately before the first time they are emitted.

[cachemeifyoucan]

Using a one way mapping from original ID to the ID in serializer is what I am thinking. I guess the usage of customized diagnostics is quite limited and it is fine just put a bit more information into the serialized format.