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

⚠️⚠️⚠️ Warnings ⚠️⚠️⚠️

• Regression or critical bug fixes should always target the stable branch. Learn more about rebasing to stable or the D release process.

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Testing this PR locally

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

[ntrel]

This makes _dupCtfe get inferred @system even though there's no cast:

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

Not sure how to avoid that. It lowers to:

for (; __key3 < __r2.length; __key3 += 1LU)
{
    const ref const(S) e = __r2[__key3];
    (ref S __appendtmp4 = e;) , (_d_arrayappendcTX(res, 1LU) , res[res.length - 1LU] = __appendtmp4 , res);
}

But still there's no cast and _d_arrayappendcTX is @trusted anyway.

[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 https://github.com/dlang/dmd/pull/16315#issuecomment-2016931657