[Issue 18469] New: [REG 2.079-b1] Segfault when trying to get type of __dtor.opCall

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18469

          Issue ID: 18469
           Summary: [REG 2.079-b1] Segfault when trying to get type of
                    __dtor.opCall
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: regression
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: b2.temp at gmx.com

this crash the compiler:

```
class Bar
{
    ~this(){}
    this(){alias T = typeof(Bar.__dtor.opCall);}
}

void main() {}
```

instead of "Error: no property 'opCall' for type 'void'", like previously.
Nonsense test case is because it is reduced from an usage of
std.traits.isCallable.

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