dap: add ability to keep ops unique when marshaling

[jsternberg]

Marshaling with the WithIdentities() constraint will ensure operations
are given unique identities to ensure they marshal with different
values. This prevents the same operation from being merged with other
identical operations.

This is useful for the debug adapter protocol which wants to know the
original locations of operations when they are in unique places.

This behavior can be enabled for dockerfiles by setting
BUILDKIT_WITH_IDENTITIES.

[tonistiigi]

I don't think this is the right approach. It is beneficial for the solver to have matching LLB digest as this is the most optimal way how the solver can deduplicate the build graph. Even if the cache keys still match, the deduplication would be added in the "merging edges" stage, that is a much more complicated codepath.

I think if we want to add more debug info to the steps, eg. that one step exists in multiple places of the source code, we should do it outside of the object that defines LLB digest, like how sourcemap is handled atm. Iirc even the name of the step does not participate in the LLB digest for that reason, so we shouldn't do it for extra debug info.

[jsternberg]

Closing this for now. I'm not a big fan of this solution and we were able to work around the issue with this PR inside of buildx. I'll keep the code around just in case that solution isn't good enough, but I would prefer not adding a hacky solution like this to the LLB if we don't really need it.