[TSan/Exclusivity] Make test more robust against optimization

[devincoughlin]

Add a "black hole" uninstrumented function to the test that prevents the
compiler from removing an unused read that needs to be instrumented by
TSan for the test to pass.

This is needed to prevent the optimizer from removing a useless read
when static enforcement of exclusivity is turned on.

[devincoughlin]

@swift-ci Please smoke test and merge.

[atrick]

Thanks! Do you understand why enabling access markers caused the read to be removed? Or did disabling the SILGen peephole actually expose another optimizations?

[devincoughlin]

It is due to disabling the peephole (it manifests whenever the peephole is removed regardless of whether the access markers are being emitted). From looking at the generated lllvm IR I saw the unused read was completely optimized away — so it wasn’t getting instrumented by the LLVM TSan pass.

[atrick]

I've noticed that disabling that peephole speeds up some code. I think it's actually better to emit canonical IR and optimize the temporary copy later.