[Issue 21034] concatenation with a string literal could also append the trailing null

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21034

--- Comment #2 from Basile-z <b2.temp at gmx.com> ---
More simply:  the result of `LHS ~ RHS` should be guaranteed to be zero
terminated when RHS is a StringExp.

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