Fix #79986: str_ireplace bug with diacritics characters

`tolower()` returns an `int`, so we must not convert to `char` which
may be `signed` and as such may be subject to overflow (actua

Fix #79986: str_ireplace bug with diacritics characters

`tolower()` returns an `int`, so we must not convert to `char` which
may be `signed` and as such may be subject to overflow (actually,
implementation defined behavior).

Closes GH-6007

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