And move the implementation of `SIL.Type.canBeClass` to the AST Type. The SIL Type just calls the AST Type implementation.
Also rename `SIL.Type.canonicalASTType` -> `SIL.Type.astType`.
* Make ExportedSourceFile hold any Syntax as the root node
* Move `ExportedSourceFileRequest::evaluate()` to `ParseRequests.cpp`
* Pass the decl context and `GeneatedSourceFileInfo::Kind` to
`swift_ASTGen_parseSourceFile()` to customize the parsing
* Make `ExportedSourceFile` to hold an arbitrary Syntax node
* Move round-trip checking into `ExportedSourceFileRequest::evaluate()`
* Split `parseSourceFileViaASTGen` completely from C++ parsing logic
(in `ParseSourceFileRequest::evaluate()`)
* Remove 'ParserDiagnostics' experimental feature: Now that we have
ParserASTGen mode which includes the swift-syntax parser diagnostics.
Add the necessary compiler-side logic to allow
the regex parsing library to hand back a set of
features for a regex literal, which can then be
diagnosed by ExprAvailabilityWalker if the
availability context isn't sufficient. No tests
as this only adds the necessary infrastructure,
we don't yet hand back the features from the regex
parsing library.
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
