This change fixes https://crbug.com/834474, a build failure caused by
LowerTypeTests not preserving .symver symbol versioning directives for
exported functions. Emit symver information to ThinLTO summary data and
then propagate symver directives for exported functions to the merged
Emitting symver information to the summaries increases the size of
intermediate build artifacts for a Chromium build by less than 0.2%.