Don't include support for old mangling in runtimes without ObjC interop

[kubamracek]

This saves a surprising amount of codesize in the runtime (~25 kB on x86_64).

[compnerd]

I could see an argument for this being something that you could control rather than associating this with the stable ABI. Id like to see Windows aim for a stable ABI at some point, and wouldn't want to include the old decoration schemes since it pre-dated the cut point. I don't think that adding in the option needs to hold up this change though.

[kubamracek]

@swift-ci please test

[mikeash]

Can we make this conditional on ObjC interop rather than the stable ABI? ObjC is why we keep the old mangling around, so that would probably make more sense. I'm fine with adjusting that later if you prefer, though.

[kubamracek]

Changed this to be conditioned on ObjC interop. But it turns out that CMake didn't really know whether ObjC interop is enabled for the platform we're building, so I've added that. Please re-review, @mikeash, @compnerd, thanks!

[kubamracek]

@swift-ci please test

[compnerd]

Well, the interesting thing is that ObjC interop is not a build time constant - it is a runtime value (-enable-objc-interop or -disable-objc-interop).

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 561621a97e03042f21ef6f2333e90458adc1a680

[kubamracek]

@swift-ci please test

[kubamracek]

preset=stdlib_S_standalone_minimal_macho_x86_64,build,test
@swift-ci please test with toolchain and preset

[kubamracek]

@swift-ci please test Windows platform

[mikeash]

It's both, right? ObjC interop is enabled or disabled in the stdlib at build time, and then you can override the default when building a program. I don't think we really support mismatched values, though. The flag seems to be there so the frontend doesn't have to embed the knowledge of which targets support ObjC interop, and for use by codegen tests.

[compnerd]

@mikeash yes, and I think that this change moves to actually clean that up. The sinking of the option to the stdlib makes this better. The demangler is in lib/ rather than stdlib/ which is what I think is making me uncomfortable (the way that things are phrased). I think that the change itself is fine now.