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Use real illegal UTF-8 byte
The code here was wrong in assuming that \xFF is not legal in UTF-8
encoded strings. It currently doesn't work due to a bug, but that may
eventually be fixed: [perl #116867]. The comments are also wrong that
all bytes are legal in UTF-EBCDIC.
It turns out that in well-formed UTF-8, the bytes C0 and C1 never appear
(C2, C3, and C4 as well in UTF-EBCDIC), as they would be the start byte
of an illegal overlong sequence.
This creates a #define for an illegal byte using one of the real illegal
ones, and changes the code to use that.
No test is included due to #116867.