[Issue 20458] CRTP + pass alias to virtual member to mixin = runtime crash

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20458

--- Comment #3 from Basile-z <b2.temp at gmx.com> ---
The same corruption without the getMember trait:

```
mixin template Impl(T){
    alias a = T.test;       
    pragma(msg, typeof(a)); // launch dsymbolsema and corrupt vtbl
}

class A(T) {
    mixin Impl!T;
}

class Foo : A!Foo {
    void test() {}
}

void main() {
    (new Foo).test();
}   
```

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