[Coverage] Collect all function records in an object (D69471 followup)

After the format change from D69471, there can be more than one section
in an object that contains coverage function records. Look up each of
these sections and concatenate all the records together.

This re-enables the instrprof-merging.cpp test, which previously was
failing on OSes which use comdats.

Thanks to Jeremy Morse, who very kindly provided object files from the
bot I broke to help me debug.