Refactor ThrowsOrReturnsNull from SonarAnalyzer.CSharp/Extensions/MethodDeclarationSyntaxExtensions.cs

[mary-georgiou-sonarsource]

ThrowsOrReturnsNull iterates the whole tree couple of times - there might be a way to optimize it.

Additionally as mentioned in a comment int the method "For simplicity this returns true for any method which contains a NullLiteralExpression but this could be a source of FNs".
This claim should be checked as it can be the source of FNs.