[NFC] Collect function names first in PossibleContents' External phase

[kripken]

The External phase considers all externally-visible things and marks them
as such. That includes functions in a public table. The old code ran on each
item and processed it, which turns out to be very slow on Dart: Dart can
have massive tables with many repeated elements, so it is much faster to
first collect the function names in a set, then process those.

This makes --gufa on this large Dart testcase 7.7x faster - not 7.7 percent,
but times: from 393 seconds to 51. (This is the same situation as in #8048,
again the huge table is not something we have handled well.)

[tlively]

Wow!