Decls with a package access level are currently set to public SIL
linkages. This limits the ability to have more fine-grained control
and optimize around resilience and serialization.
This PR introduces a separate SIL linkage and FormalLinkage for
package decls, pipes them down to IRGen, and updates linkage checks
at call sites to include package linkage.
Resolves rdar://121409846
The relationship between the code in these two libraries was fundamentally circular, indicating that they should not have been split. With other changes that I'm making to remove circular dependencies from the CMake build graph I eventually uncovered that these two libraries were required to link each other circularly, but that had been hidden by other cycles in the build graph previously.
