Adapt QNX support to the introduction of LINKER_DRIVER

[jakepetroules]

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[jakepetroules]

I'm not thrilled with how QNX leaks out of its plugin. I think LINKER_DRIVER can't be an enum and this has to be done in a somewhat more flexible way instead. Any particular ideas?

Maybe LINKER_DRIVER can be a plain string, and we set it to clang by default in Settings.swift. For LD_OPTIMIZATION_LEVEL I think we need something like LINKER_DRIVER_IS_GCC_COMPATIBLE and maybe an extension point that returns a list of gcc-compatible linker drivers?

[dschaefer2]

I'm not thrilled with how QNX leaks out of its plugin.

I'm not thrilled swift-build needs to know about QNX. I look forward to adding it to the Swift SDK story and how swift-build can deal with new platforms it's never heard about :).

[jakepetroules]

We'll see. Swift SDKs can only go so far, even the compiler has to have knowledge of specific platforms to a certain point.

Keep in mind part of the motivation here is to identify what sort of things actually need to become configurable to support various different platforms.