As the optimizer uses more and more AST stuff, it's now time to create an "AST" module.
Initially it defines following AST datastructures:
* declarations: `Decl` + derived classes
* `Conformance`
* `SubstitutionMap`
* `Type` and `CanonicalType`
Some of those were already defined in the SIL module and are now moved to the AST module.
This change also cleans up a few things:
* proper definition of `NominalTypeDecl`-related APIs in `SIL.Type`
* rename `ProtocolConformance` to `Conformance`
* use `AST.Type`/`AST.CanonicalType` instead of `BridgedASTType` in SIL and the Optimizer
Make SILLInkage available in SIL as `SIL.Linkage`.
Also, rename the misleading Function and GlobalVariable ABI `isAvailableExternally` to `isDefinedExternally`
The OSSA elimination pass has not yet been moved below all high level
function passes. Until that work has been completed the Autodiff
closure-spec optimization pass cannot solely support OSSA instructions.
