SE: Learn ObjectConstraint for nullable types

[pavel-mikula-sonarsource]

Add Null constraint to the Roslyn SE for nullable value types.

• Copy known constraints from nullable value to .Value member
• Set BoolConstraint on .HasValue from known ObjectConstraint, and do not learn NotNull from property reference
• Set Null constraint for new Nullable<int>()
• Set argument constraints for new Nullable<int>(value) or new Nullable<int>(42)
• Explicit and implicit conversions from/to nullable and non-nullable
• Set branching ObjectConstraint when branching on .HasValue
• Set branching ObjectConstraint when branching on null checks
• Set branching ObjectConstraint when branching on ==value checks
• Support .GetValueOrDefault() - do not lean "NotNull" on the invocation, propagate constraint
• Lean from RecursivePattern for nullable types
• Set ObjectConstraint when on Debug.Assert tests
• Review SE - Non-nullable value types: Add NonNullableValueTypeCheck #6864 (comment)
• Take a look at b = x.HasValue & x.Value;
• Check .Equals(42) and .Equals(null), object.Equals(left, right)

#6840 deals with S2259 overlap

This issue requires more investigation to implement properly.

[pavel-mikula-sonarsource]

Having just ObjectConstraint might be enough

int? i = null; // Null?
i.Value    //-> throws
i.GetHashCode()    //-> throws
i.HasValue // -> it's fine
i.GetValueOrDefault() //-> --//--
new Nullable<int>() // -> null constraint
new Nullable<int>(42) // -> not null constraint

We need to look into explicit and implicit conversions between nullable and non-nullable.
Lifting operators, consider #4250

[pavel-mikula-sonarsource]

Testing if(value.HasValue) needs to learn NotNull or Null constraints if not present.
Testring if(value == something) will be tricky

Debug.Assert(CurrentToken.Kind == SyntaxKind.Keyword && result.HasValue); // Can learn NotNull
            if (_keywordParserMap.TryGetValue(result.Value, out var handler)) // Compliant