[api-digester] Update classic ABI checker exception list

Author: lorentey

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

@@ -817,4 +817,6 @@ Func _SliceBuffer.withUnsafeBufferPointer(_:) has mangled name changing from 'Sw
 Func _SliceBuffer.withUnsafeMutableBufferPointer(_:) has been renamed to Func __abi_withUnsafeMutableBufferPointer(_:)
 Func _SliceBuffer.withUnsafeMutableBufferPointer(_:) has mangled name changing from 'Swift._SliceBuffer.withUnsafeMutableBufferPointer<A>((Swift.UnsafeMutableBufferPointer<A>) throws -> A1) throws -> A1' to 'Swift._SliceBuffer.__abi_withUnsafeMutableBufferPointer<A>((Swift.UnsafeMutableBufferPointer<A>) throws -> A1) throws -> A1'
 
+Struct String.Index has added a conformance to an existing protocol CustomDebugStringConvertible
+
 // *** DO NOT DISABLE OR XFAIL THIS TEST. *** (See comment above.)