We only want to report signature clashes for declarations that come
from the module currently being serialized. In GlobalDeclarationTable,
declarations from other modules could also be stored.
Checking whether a declaration is a Lazy IR declaration to
determine if it comes from an external module works okay, but it is
a hack which relies on an implementation detail of IR, which may or
may not work in the future.
Use a more robust logic here, since IrFileSerializer is always aware
which declarations are declared in the current module and which are
just referenced from it.
Sergej Jaskiewicz
06684f207a