[Issue 21805] Allow trinary expression in types

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21805

--- Comment #1 from Bolpat <qs.il.paperinik at gmail.com> ---
Obvious semantics:

ConditionalType. The OrOrExpression is evaluated at compile-time. If it
evaluates to `true`, the Type expression results to the given BasicType, if it
evaluates to `false`, it results to the ConditionalType. If the evaluation
fails, it is an error.

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