[Issue 20663] writeln should be @nogc

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20663

--- Comment #2 from Manu <turkeyman at gmail.com> ---
Dunno what's going on with DIP1008, but I actually agree that using the
replacement char is actually the more interesting and useful thing to do.

I might be interested in throwing a UTF exception from a function like
`validateString`, but I don't want writeln to throw! I want it to write text to
the output stream... it has one job!

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