coverage, propagate visibility

[Nathan Sidwell]

This patch propagates visibility information in the same way that weakness is 
for externally visible coverage objects.  I don't know whether this will fix the 
reported firefox build failure -- can you try?  Even if it doesn't, I'm pretty 
sure it's an improvement, but I'll leave it for comment before committing eitherway.

built & tested on i686-pc-linux-gnu.

nathan
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