[Issue 24717] alias edge cases with tupleof

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24717

--- Comment #3 from Nick Treleaven <nick at geany.org> ---
> should be implemented as a symbol sequence of implicit ref declarations

Actually `AliasSeq!(__si, __sc)` can't be the lowering because then
`__traits(identifier, s.tupleof[0])` would be `__si`, not `i`. But conceptually
it should be a superset of an lvalue sequence.

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