[linker] Mark SecurityDeclarations [#28918]

[linker] Mark SecurityDeclarations [#28918]

Security declarations looks very similar to custom attributes - but
they are not (e.g. a different metadata table).

They need to be marked, just like other metadata, by the linker because
they can encode types that won't show up in the type (or assembly)
references.

Ignoring this can create an invalid binary (the kind that even Cecil
can't load back for further processing).