[SE-0489][abi] add new conformances to ABI stability tests.

Author: ZevEisenberg

test/api-digester/stability-stdlib-abi-without-asserts.test

@@ -874,4 +874,8 @@ Func type(of:) has been removed
 Func _float32ToStringImpl(_:_:_:_:) is a new API without '@available'
 Func _float64ToStringImpl(_:_:_:_:) is a new API without '@available'
 
+// New conformances from SE-0489: Better debugDescription for EncodingError and DecodingError
+Enum DecodingError has added a conformance to an existing protocol CustomDebugStringConvertible
+Enum EncodingError has added a conformance to an existing protocol CustomDebugStringConvertible
+
 // *** DO NOT DISABLE OR XFAIL THIS TEST. *** (See comment above.)