Swift: collapse TypeRepr hierarchy

Now `TypeRepr` is a final class in the AST, which is more or less just
a type with a location in code.

As the frontend does not provide a direct way to get a type from a
type representation, this information must be provided when fetching
the label of a type repr.

This meant:
* removing the type repr field from `EnumIsCaseExpr`: this is a virtual
  AST node introduced in place of some kinds of `IsEpxr`. The type
  repr is still available from the `ConditionalCheckedCastExpr` wrapped
  by this virtual node, and we will rebuild the original `IsExpr` with
  the IPA layer.
* some logic to get the type of keypath roots has been added to
  `KeyPathExpr`. This was done to keep the `TypeRepr` to `Type` relation
  total in the DB, but goes against the design of a dumb extractor. The
  logic could be moved to QL in the future
* in the control flow library, `TypeRepr` children are now ignored. As
  far as I can tell, there is no runtime evaluation going on in
  `TypeRepr`s, so it does not make much sense to have control flow
  through them.

swift/codegen/schema.yml

@@ -7,8 +7,7 @@ _includes:
 _directories:
   decl: Decl$|Context$
   pattern: Pattern$
-  type: Type$
-  typerepr: TypeRepr$
+  type: Type(Repr)?$
   expr: Expr$
   stmt: Stmt$
 
@@ -181,6 +180,7 @@ Stmt:
 
 TypeRepr:
   _extends: AstNode
+  type: Type
 
 FunctionType:
   _extends: AnyFunctionType
@@ -391,7 +391,6 @@ EnumIsCaseExpr:
   _extends: Expr
   _children:
     sub_expr: Expr
-    type_repr: TypeRepr
     element: EnumElementDecl
 
 ErrorExpr:
@@ -442,7 +441,7 @@ KeyPathDotExpr:
 KeyPathExpr:
   _extends: Expr
   _children:
-    parsed_root: Expr?
+    root: TypeRepr?
     parsed_path: Expr?
 
 LazyInitializerExpr:
@@ -1162,87 +1161,3 @@ FloatLiteralExpr:
 IntegerLiteralExpr:
   _extends: NumberLiteralExpr
   string_value: string
-
-ErrorTypeRepr:
-  _extends: TypeRepr
-
-AttributedTypeRepr:
-  _extends: TypeRepr
-
-IdentTypeRepr:
-  _extends: TypeRepr
-
-ComponentIdentTypeRepr:
-  _extends: IdentTypeRepr
-
-SimpleIdentTypeRepr:
-  _extends: ComponentIdentTypeRepr
-
-GenericIdentTypeRepr:
-  _extends: ComponentIdentTypeRepr
-
-CompoundIdentTypeRepr:
-  _extends: IdentTypeRepr
-
-FunctionTypeRepr:
-  _extends: TypeRepr
-
-ArrayTypeRepr:
-  _extends: TypeRepr
-
-DictionaryTypeRepr:
-  _extends: TypeRepr
-
-OptionalTypeRepr:
-  _extends: TypeRepr
-
-ImplicitlyUnwrappedOptionalTypeRepr:
-  _extends: TypeRepr
-
-TupleTypeRepr:
-  _extends: TypeRepr
-
-CompositionTypeRepr:
-  _extends: TypeRepr
-
-MetatypeTypeRepr:
-  _extends: TypeRepr
-
-ProtocolTypeRepr:
-  _extends: TypeRepr
-
-OpaqueReturnTypeRepr:
-  _extends: TypeRepr
-
-NamedOpaqueReturnTypeRepr:
-  _extends: TypeRepr
-
-ExistentialTypeRepr:
-  _extends: TypeRepr
-
-PlaceholderTypeRepr:
-  _extends: TypeRepr
-
-SpecifierTypeRepr:
-  _extends: TypeRepr
-
-InOutTypeRepr:
-  _extends: SpecifierTypeRepr
-
-SharedTypeRepr:
-  _extends: SpecifierTypeRepr
-
-OwnedTypeRepr:
-  _extends: SpecifierTypeRepr
-
-IsolatedTypeRepr:
-  _extends: SpecifierTypeRepr
-
-CompileTimeConstTypeRepr:
-  _extends: SpecifierTypeRepr
-
-FixedTypeRepr:
-  _extends: TypeRepr
-
-SilBoxTypeRepr:
-  _extends: TypeRepr