S3655: Honor null forgiving operator

[antonioaversa]

Improves #6794
Follows discussion had here.

When the user specifies the null forgiving operator on a value access of a Nullable<T> (e.g. nullable!.Value), no issue is raised by S3655, even when a path is found, in which nullable is null.

It also removes false positives encountered during peach validation for #6794, such as

Label? after = null;
if (info.Is64BitSwitch)
{
    after = _ilg.DefineLabel();
    // ...
}
// ...
if (info.Is64BitSwitch)
{
    _ilg.MarkLabel(after!.Value); // <- This should be compliant
}

source: dotnet-runtime:src/libraries/System.Linq.Expressions/src/System/Linq/Expressions/Compiler/LambdaCompiler.Statements.cs

[antonioaversa]

@zsolt-kolbay-sonarsource
As discussed offline, the local function name has been changed to NotNullFlowState from NoNullForgiving, since Roslyn flow analysis can deduce a non-null nullability state even in absence of the null forgiving operator.
On the other hand, when we reach this condition of the if statement, we are in a very narrow scenario, involving a nullable value type, and it's not clear to me how that would happen.

[zsolt-kolbay-sonarsource]

LGTM

[pavel-mikula-sonarsource]

LGTM with one refactoring to do before merging

[pavel-mikula-sonarsource]

The name seems wrong to me from looking at the implementation.
Implementation says (equivalent evolution on each line)

FlowState != NotNull
FlowState Not NotNull
Not NotNull FlowState

while the method is named

NotNull FlowState

so it's the opposite.

It's should be NotNotNullFlowState and that's bad.

I don't have better idea for a good name. So we can avoid it like this:

        &&  FlowState(reference.Instance) != NullableFlowState.NotNull
// ...
        NullableFlowState FlowState(IOperation reference) =>
            SemanticModel.GetTypeInfo(reference.Syntax).Nullability().FlowState;

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