This information will be necessary when a change to a file forces
summaries to be relinked.
Note that the way we determine the prefix is a bit kludgy: we look at
the export namespaces of the imported libraries and arbitrarily choose
a prefix that points to the given element. This will give subtly
incorrect results if a name is reachable via multiple prefixes,
however this is unlikely to pose a problem in non-contrived use cases.
See the test
`fail_type_reference_to_type_visible_via_multiple_import_prefixes` for
a more detailed explanation of what might go wrong.
R=scheglov@google.com
Review URL: https://codereview.chromium.org/1574763002 .
Paul Berry
8a29a99286