[scudo] Replace eraseHeader with compareExchangeHeader for Quarantined chunks

[scudo] Replace eraseHeader with compareExchangeHeader for Quarantined chunks

Summary:
The reason for the existence of `eraseHeader` was that it was deemed faster
to null-out a chunk header, effectively making it invalid, rather than marking
it as available, which incurred a checksum computation and a cmpxchg.

A previous use of `eraseHeader` was removed with D50655 due to a race.

Now we remove the second use of it in the Quarantine deallocation path and
replace is with a `compareExchangeHeader`.

The reason for this is that greatly helps debugging some heap bugs as the chunk
header is now valid and the chunk marked available, as opposed to the header
being invalid. Eg: we get an invalid state error, instead of an invalid header
error, which reduces the possibilities. The computational penalty is negligible.

Reviewers: alekseyshl, flowerhack, eugenis

Reviewed By: eugenis

Subscribers: delcypher, jfb, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D51224

llvm-svn: 340633