[Issue 21740] Typeof mixin regression with v2.096

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21740

--- Comment #11 from kinke <kinke at gmx.net> ---
Ah, so the point is to allow stuff like `mixin((() => "1")(), " + ", 2)` too:
https://dlang.org/spec/expression.html#mixin_expressions

So with void expressions now being represented as "cast(void)0" strings instead
of "<void>", there's a chance that a previously invalid mixin with some void
expression argument doesn't fail anymore.

Do type arguments make a lot of sense for mixin expressions? `mixin("int") d =
3;` works fine; `mixin(int) d = 3;` yields:

Error: found `)` when expecting `.` following int
Error: found `;` when expecting `)`
Error: no identifier for declarator `mixin((int).d = 3)`

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