Add CastExp.toLvalue dip1000 error if implicit pointer conversion would fail

[ntrel]

From https://dlang.org/spec/expression.html#.define-lvalue

The following expressions, and no others, are called lvalue expressions or lvalues
...
cast(U) expressions applied to lvalues of type T when T* is implicitly convertible to U*;
cast() and cast(qualifier list) when applied to an lvalue.

The first rule was not enforced by dmd.
The second rule contradicts the point of the first rule. It should perhaps could be updated to work in the same way.

Fixes Issue 23530 - casting immutable away allowed in safe.

Edit: Changed to dip1000 error rather than obsolete warning.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	23530	critical	casting immutable away allowed in safe

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Testing this PR locally

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dub run digger -- build "master + dmd#15597"

[ntrel]

I'm not sure where the cast expression comes from when calling _dupCtfe:

        foreach (ref e; a)
            res ~= e; // offending line

I tried to work around that by ignoring any casts in functions being interpreted at compile-time, but I'm clearly not detecting that correctly. Using SCOPE.ctfeBlock doesn't work either.

[ntrel]

Closing in favour of #16315 (comment)