Enable library evolution in CMake build

[dabrahams]

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The use of @_implementationOnly causes the compiler to warn of instability at runtime otherwise.

[compnerd]

CC: @rauhul

[rauhul]

Thoughts on this approach instead: https://github.com/apple/swift-argument-parser/pull/666

I don't really see a need to use this attribute anymore. It's really just doesn't work without library evo enabled anyways.

[rauhul]

superseded by: #666