[JITLink] Update external symbol scopes to reflect scopes of resolved defs.

This is a counterpart to ffe2dda2, and does for scope what that commit did
for linkage.

Making the scope of external definitions visible to JITLink plugins will
allow us to distinguish hidden weak defs (which do not need to be tracked by
default) from default-scoped weak defs (which need to be updated to point at
a single chosen definition at runtime).