* move it from the SIL to the AST module (where it belongs)
* change the signature of `diagnose` from `diagnose(location, .some_error)` to `diagnose(.some_error, at: location)`
* add an overload to allow passing a `SIL.Location` directly to `diagnose`
* add a `Diagnostic : Error` utility struct which allows throwing a `Diagnostic`
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
