Add factory methods for different types of LinkageLocations, and ensure that
accesses to the underlying data in the location are classified by type and
funneled through explicit accessors.

Also change the representation of LinkageLocation to use a BitField rather
than using a reserved section of the integer range.

Review URL: https://codereview.chromium.org/1262343002

Cr-Commit-Position: refs/heads/master@{#29938}