[Issue 21849] UTF8: Loc column numbers don't respect multi-column characters

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21849

--- Comment #1 from Iain Buclaw <ibuclaw at gdcproject.org> ---
Reason is that column tracking just uses simple pointer subtraction.

https://github.com/dlang/dmd/blob/db13c6c948b4724ad8235c7063feba1241f685b4/src/dmd/lexer.d#L2291

This works when a line only consists of byte characters.

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