Add visit method for UnionDeclaration in transitivevisitor

[lucica28]

UnionDeclaration should have it's own visit method because one might want to extend the transitive visitor class and override the visit method of StructDeclaration, but keep the default logic for UnionDeclaration. Without a visit method for UnionDeclaration, what would happen currently in this situation is that if you override the visit method for StructDeclaration you automatically override also the visit method for UnionDeclaration

[dlang-bot]

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Testing this PR locally

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dub run digger -- build "master + dmd#14453"

[ibuclaw]

Small suggestion, don't merge, rebase.

[pbackus]

CI failure is because you need to update the C++ headers to match the D code. From the log:

The file src/dmd/frontend.h seems to be out of sync. This is likely because changes were made which affect the C++ interface used by GDC and LDC.

Make sure that those changes have been properly reflected in the relevant header files (e.g. src/dmd/scope.h for changes in src/dmd/dscope.d).

To update frontend.h and fix this error, run the following command:

src/build.d cxx-headers-test AUTO_UPDATE=1

Note that the generated code need not be valid, as the header generator (src/dmd/dtoh.d) is still under development.

To read more about frontend.h and its usage, see src/README.md#cxx-headers-test

[lucica28]

Small suggestion, don't merge, rebase.

thank you for the suggestion