Remove string mixin from _d_arrayappendcTX

[dkorpel]

It was introduced in #2632, but the two branches look identical to me.

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[thewilsonator]

while we're changing this, please move this pragma outside of the function

[maxhaton]

Why have it at all

[thewilsonator]

otherwise looks good

[RazvanN7]

cc @teodutu . Does this affect your current work in any way?

[teodutu]

@RazvanN7, no. nothrow was added because at that time it wasn't inferred properly, but now it seems to be fine.

[RazvanN7]

Maybe a rebase will fix the vibe.d failure?

[RazvanN7]

Vibe failure is unrelated, so merging this as is.

[dkorpel]

It looks related to me:

core.exception.AssertError@src/dmd/dinterpret.d(4878): CTFE cannot interpret _d_arrayappendcTX!

I think the pragma(inline, false); was actually needed.

[RazvanN7]

Hmm, I couldn't see that error message. Maybe it's required that the function body is a string so that ctfe can interpret it?