`String(describing:)` does a bunch of dynamic casts
that can be pretty slow. Use interpolation instead,
which bypasses them.
For `swift-frontend`, this brings the time taken
for type-checking an empty file down from ~100ms
to ~70ms.
For `swift build`, this brings the time taken for
a null build down from ~600ms to ~450ms (the
larger delta is presumably due to the fact that
there's much more Swift code in `swift-package`).
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
