Clang function types v2: Electric Boogaloo (parsing + printing)

[varungandhi-apple]

First three commits are from #28726. Tried to get it out soon for review. Some things are still TODO but the core things are done:

• Add some test cases to check that diagnostics trigger. I tested manually and it seemed to work, but still.
• Add some test cases for SIL printing. AST and SIL logic are very close, so this should be minimal work. We can't add test cases for this right now, because we aren't storing the type in SIL
• Add some test cases when the Clang type is coming from the ClangImporter. Technically, that shouldn't have any effect, but still.
• Add some complex test cases using typedefs, struct pointers etc?

[varungandhi-apple]

We should run still run the tests now though.

@swift-ci please test

[varungandhi-apple]

Still haven't had the time to write the extra tests, will do that soon 😐.

@swift-ci please test

[varungandhi-apple]

Tests are kinda' done. Seems minimal, but there's only so many things you can do with type declarations, I guess. When we return a function pointer through a typealias, then the type of the variable is ctypes.fptr?, without any @convention, so we don't have anything to test there.

[varungandhi-apple]

@swift-ci please test

@swift-ci please test source compatibility

[varungandhi-apple]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - b3247951dff767d46fb119d9629683b8ebfd3be1

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - b3247951dff767d46fb119d9629683b8ebfd3be1

[varungandhi-apple]

@rjmccall LGTY?

[varungandhi-apple]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 20ee1594da9531067fba601292afa559478f9f39

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 20ee1594da9531067fba601292afa559478f9f39

[varungandhi-apple]

@swift-ci please smoke test

[rjmccall]

LGTM