[ClangImporter] Predefine __swift__ in imported (Obj)C code.

[jrose-apple]

• Explanation: Provides a macro allowing imported code to distinguish between Swift versions, particularly between Swift 2 and Swift 3. More explanation and rationale in https://lists.swift.org/pipermail/swift-dev/Week-of-Mon-20160822/002754.html. While this is hardly a critical Swift 3 feature, it's much more effective if it goes in now rather than in a point release, and so if there are any further critical fixes we should take this too.
• Scope: Pretty much affects no one unless they were defining this macro manually. It does also affect the implementation of __SWIFT_COMPILER_VERSION, which is used only for internal processes at Apple, but the intent is that nothing changes there either.
• Issue: rdar://problem/26921435
• Reviewed by: @DougGregor, @bitjammer
• Risk: Very low.
• Testing: Added a compiler regression test.

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[jrose-apple]

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[jrose-apple]

P.S. If anyone's wondering what's up with __SWIFT_COMPILER_VERSION and why we don't just use that, it's because self-built toolchains and development snapshots don't have Apple build numbers, and therefore it doesn't make sense to write logic checking Apple build numbers.