DNM: restore error in swift build

Author: compnerd

include/swift/AST/Types.h

@@ -352,9 +352,6 @@ enum class TypeMatchFlags {
 };
 using TypeMatchOptions = OptionSet<TypeMatchFlags>;
 
-// Forward declare the ErrorType as otherwise we will match `lldb::ErrorType`.
-class ErrorType;
-
 /// Base class for all types which describe the Swift and SIL ASTs.
 ///
 /// See TypeNodes.def for a succinct description of the full class hierarchy.