replace Control.Concurrent.QSem

Right now there's no replacement for packages in the HP (e.g. Cabal), leading to all kinds of ugly workarounds. See https://github.com/haskell/cabal/pull/1108 for some of the pain this causes.

If QSem is broken we should fix it, not deprecate it and let users deal with the difficulty of finding a correct semaphore implementation that they can use.

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
Test case
Differential revisions
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Blocking
CC
Operating system
Architecture