NFC: Make SILType.h and SILDeclRef.h not depend on SIL/*.h

[davezarzycki]

SILType and SILDeclRef do not actually need anything from SIL/*.h. Also, a few dependencies can be pushed out of the headers into cpp files to speed up incremental rebuilds.

[davezarzycki]

@swift-ci please test

[davezarzycki]

@jckarter – After this clean up, I'd like propose moving SILType.h from include/swift/SIL to include/swift/AST. This would simplify a bunch of things and allow for easier self-consistency checks in the SIL specific types of the AST type system.

[jckarter]

In principle, I feel like SILType-specific consistency checks could go in the SIL library too. It would be nice if we could drive a stronger wedge between SIL types and AST types, to help make it harder to confuse lowered types and formal types when working in SILGen, optimization passes, and IRGen, instead of making them more similar.

[davezarzycki]

Okay. Let me think about that. In any case, this pull request is still worthwhile, right? This is just header dependency clean up.

[jckarter]

Sure, this patch looks good.