[Issue 23153] New: Immutable variables should undergo same flow analysis in module constructors as in regular constructors

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23153

          Issue ID: 23153
           Summary: Immutable variables should undergo same flow analysis
                    in module constructors as in regular constructors
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: ibuclaw at gdcproject.org

i.e: This compiles:
```
immutable int x;
immutable int* y;
shared static this()
{
    y = &x;
    assert(*y == 0);
    x = 42;
    assert(*y == 42);
}
```

But the expected behaviour is to fail compilation, same as:
```
struct S
{
    immutable int x;

    this(int n)
    {
        auto y = &x;
        assert(*y == 0);
        x = n;
        assert(*y == n);
    }
}
```

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