Linker warnings on Windows with Swift Argument Parser

[compnerd (Saleem Abdulrasool)]

@rauhul

Do you have a sketch of what an example Package.swift and resulting .build directory would look like with the changes proposed? I don't think I'm following the idea to drop: ".library(name:type:targets:) where type is .dynamic"

I think that a Package.swift change would be something along the lines of what I describe at:

Migration Plan: swift-corelibs-foundation to swift-foundation

I think the simplest thing would require completely rearchitecting llbuild/SPM because the build model doesn't allow for n-parallel builds (with different flags) of each target, and the SPM manifest as well as that does not explicitly state the type of linkage per target. The object files must be separated so the contents of .build need to be restructured as well. That is to say, we need to change to a model as the following: .target("a", type: .dynamic_library, dependencies: [ .target("b", …

I don't have a strong opinion on the .build structure - each target would need to be duplicated though - static or dynamic variant.

@rauhul

For example I use this to explicitly build SVD2LLDB (an lldb plugin) as a dylib, instead of whatever a client might want.

I agree that it is useful, however, if we build everything for static and only for static, then we cannot build a dynamic library. I do not think that it is reasonable to restrict dynamic library products to non-Windows targets only, so the equitable solution here is to simply drop all support for dynamic library products. I fully agree that this makes the build system something which would be very limiting for many use cases.