[DNM][AST] Keep track of an "isDerivableFromSwiftType" bit for Clang types.

[varungandhi-apple]

I wasn't sure what to name the type, so I've named it ClangTypeWrapper for now.

Probably clean up the PR a little bit tomorrow but I figured I should put it up sooner rather than later for feedback and testing.

[varungandhi-apple]

@swift-ci please test

[varungandhi-apple]

@swift-ci please test source compatibility

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - f413f36

[varungandhi-apple]

@swift-ci please clean test

[varungandhi-apple]

There's some other breakage because of the SIL stuff, marking this as DNM right now.

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - f413f36

[varungandhi-apple]

The behavior for marking something "isDerivable" in this PR is not correct. Arguably, "isDerivable" is the wrong name altogether. We do not want the mangling for existing code to change. However, since multiple C types are mapped to the same Swift type, some function types like void (unsigned long) will get marked as "not derivable" because unsigned long -> Int -> long. We should instead check for something like "does this function type have additional attributes".

[varungandhi-apple]

I've opened #34057 with a more recent version of these changes, so I'm closing this PR.