llama : better replace_all

[ggerganov]

fix #8841

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[ngxson]

LGTM. Actually I copied the original function somewhere in the internet without thinking much about it (we always pass a const string for search anw, so at least for now search is never empty)

[LostRuins]

There's also another duplicate of the original function at https://github.com/ggerganov/llama.cpp/blob/master/src/llama-vocab.cpp#L19 which you might wish to replace as well.