[Typed throws] Map the experimental feature flag over to the new parser's flag

Author: DougGregor

lib/ASTGen/Sources/ASTGen/SourceFile.swift

@@ -36,6 +36,7 @@ extension Parser.ExperimentalFeatures {
       }
     }
     mapFeature(.ThenStatements, to: .thenStatements)
+    mapFeature(.TypedThrows, to: .typedThrows)
   }
 }