Move FuncDeclaration.addPreInvariant and FuncDeclaration.addPostInvariant to funcsem

[elshorbagyx]

To break the dependency of func.d on objc.d

[dlang-bot]

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[thewilsonator]

these functions are only called from semantic3.d so if you move them there you can make them private.

[elshorbagyx]

addPostInvariant is used in expressionsem