IRGen: disable class fast casting on Windows

[eeckstein]

It causes some problems on CI.

[eeckstein]

@swift-ci smoke test

[compnerd]

Can you please link the radar that tracks this? I don't want this to slip through the cracks

[compnerd]

Thinking a bit more, would it make more sense to make this Darwin vs non-Darwin? That way we can have some sort of comparison point between platforms. I know that @shahmishal has been pushing for more homogenous builds.

[compnerd]

Actually, this disables it on the Windows host - we should disable this by target configuration and for !Apple targets.

[eeckstein]

@compnerd you are right, it needs to be checked for the target. I changed that and added a link to the Jira.
I don't want to disable it for Linux, though, because linux tests didn't show any problem.

[eeckstein]

@swift-ci smoke test

[compnerd]

I think that this really does deserve a radar - Windows is a supported platform and disabling optimizations without any plans to re-enable them is not going to help Swift scale up. Could you also please include any notes from your investigation on the failure?

[compnerd]

@swift-ci please test Windows platform

[compnerd]

@swift-ci please build toolchain Windows platform

[compnerd]

@swift-ci please build toolchain Windows platform

[atrick]

I added investigation notes to the SR: https://bugs.swift.org/browse/SR-16112

[compnerd]

If it is reasonable, I think that #42286 might be preferable - it keeps the optimization enabled.