Suppress the "read-modify-write" error if type is a struct or a class

[ErnyTech]

If a struct or class with shared type implements the opUnary or opOpAssign
operator then it is possible to perform "read-modify-write" operations
because the operator's implementation is supposed to perform atomic operations.

The purpose of this modification is to allow the creation of wrappers
(with structs or classes) to run atomic operations silently, for example:

shared struct Atomic {
    int a;

    int opUnary(string s)() if (s == "++")
    {
        import core.atomic : atomicOp;
        return atomicOp!"+="(a, 1);
    }
}

Atomic atomicvar;
atomicvar++; // Safe! Atomic struct implements opUnary

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Testing this PR locally

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dub run digger -- build "master + dmd#10281"

[Geod24]

No test

[Geod24]

That doesn't look right. It's blindly allowing struct / class

[ErnyTech]

If the class/struct does not implement the operator the compiler will give the error "is not a scalar"

dmd/src/dmd/expressionsem.d

Line 5574 in d9826c8

if (exp.e1.checkScalar() ||

So I don't think further checks are necessary

[ErnyTech]

Added compilable test

Compilable test added.

[thewilsonator]

Searching for tabs
./changelog/pending.dd:321:	int opUnary(string s)() if (s == "++")
./changelog/pending.dd:322:	{
./changelog/pending.dd:323:		import core.atomic : atomicOp;
./changelog/pending.dd:324:		return atomicOp!"+="(a, 1);
./changelog/pending.dd:325:	}

[Geod24]

Why does this need to be shared if C is a shared class ?

[ErnyTech]

Using shared in the class declaration does not seem to make the class type "implicitly" shared unlike what it does in a struct

[ErnyTech]

I realized that the formatting has problems (badly converted tabs), I have to solve it

[ErnyTech]

I realized that the formatting has problems (badly converted tabs), I have to solve it

Fixed