decodeUtf8With: Undefined behaviour using non-BMP error-handler replacements

[hvr]

This issue is intended to track progress on supporting non-BMP replacement characters in the decodeUtf8With function.

See also comment at ab90c65#diff-1c42d239accd16adcae29f591c519ac6R172

Short History:

1. 2189300 introduces the first version of decodeUtf8With affected by this issue
2. efa8732 decodeUtf8With gets rewritten while retaining this this issue
3. 1780bd7 decodeUtf8With gets changed to ensure that surrogate code-points get remapped to U+FFFD
4. ab90c65 decodeUtf8With gets changed to fail non-silently in case of non-BMP replacement characters

[Bodigrim]

This has been fixed in course of UTF-8 migration.