[CAS] Add caching diagnostic processor

[cachemeifyoucan]

Add a CachedDiagnosticsProcessor that is a DiagConsumer can capture all the diagnostics during a compilation, serialized them into CAS with a format that can be replayed without re-compiling.

This is also tested via default all the diagnostics to capture and replayed via CachedDiagnosticsProcessor and pass all the tests.

[cachemeifyoucan]

@swift-ci please smoke test

[benlangmuir]

This is also tested via default all the diagnostics to capture and replayed via CachedDiagnosticsProcessor and pass all the tests.

To clarify, is this something that is always happening, or something you did manually?

[cachemeifyoucan]

This is also tested via default all the diagnostics to capture and replayed via CachedDiagnosticsProcessor and pass all the tests.

To clarify, is this something that is always happening, or something you did manually?

Manually, we can also leave a switch in there for testing purpose. There isn't a good way to dup all tests with a different configuration to have this always on and probably very expensive.

[benlangmuir]

Okay, I would like to see more explicit test coverage then for different kinds of diagnostics (notes, errors, educational notes, if there are any "stack" output similar to clang's include stacks), different output format (.diag file, stderr, different formatting options for the stderr output).

[cachemeifyoucan]

@swift-ci please smoke test

[cachemeifyoucan]

@swift-ci please smoke test

[cachemeifyoucan]

@swift-ci please smoke test

[cachemeifyoucan]

ping