[patch] Refactor the Used flag logic (and a small bug fix)

[Rafael Espíndola]

> Sounds good, go ahead.

OK. I have committed the fix. The attached patch is a rebased version
with just the refactoring.

On the testcase with 16384 redeclarations clang -cc1 goes from
0m3.412s to 0m4.759s.
On the file from Postgres in pr10651 it goes from 0m0.604s to 0m0.716s.

So it looks like at least for now we cannot afford it :-(

Cheers,
Rafael
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