[Issue 18973] New: @disable on const toHash causes unresolved symbol error

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18973

          Issue ID: 18973
           Summary: @disable on const toHash causes unresolved symbol
                    error
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: minor
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: elpenguino+D at gmail.com

Annotating const toHash methods with @disable causes unresolved symbol errors
in the link stage.

```
struct X {
    @disable size_t toHash() const;
}
```

will cause

```
onlineapp.o:onlineapp.d:_D22TypeInfo_S9onlineapp1X6__initZ: error: undefined
reference to '_D9onlineapp1X6toHashMxFZm'
```

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