Fix affinetransform init

[parkera]

What's in this pull request?

The migrator is changing NSAffineTransform() into AffineTransform(), but because AffineTransform is actually a renamed C struct (previously known as NSAffineTransformStruct), its initializer sets everything to zero.

This change removes that renaming and establishes AffineTransform as its own struct type, where we can control the behavior of the init function. The default initializer now returns an identity AffineTransform as expected.

Resolved bug number

rdar://problem/27793851 AffineTransform() and NSAffineTransform() initializers are not semantically equivalent

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[parkera]

Ugh, wrong branch. Will fix in a minute.

[parkera]

@swift-ci please test and merge

[tkremenek]

@swift-ci test

[tkremenek]

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