`is` expression evaluating `const` expression should be considered constant

^{Originally posted by **Rekkonnect** January 28, 2023}
## Summary

Despite being provided a constant expression, using an `is` pattern is not considered constant. This can be safely adjusted. As an added bonus, a warning is generated by code flow analysis saying that the expression always evaluates to `true` or `false`.

## Motivation

Conditional compilation symbols, reliance on runtime constants deriving from conditional statements

## Description

You cannot assign an `is` expression on constant LHS value to a constant field. For example,
```csharp
public const int A = 4;
public const bool B = A is 4; // illegal, despite the result being available for constant evaluation
public const bool C = A is not 3 and < 1 or 5; // illegal too
```

In the example above, `A is 4` is `A == 4`, but is not treated as such. `A == 4` passes, `A is 4` does not. My personal preference is using `is` wherever available, especially on enum constants, which also allows for extensibility of the pattern.

Since patterns on the RHS of the pattern matching expression are always constant (as of now), the only limitation is the LHS also being a constant expression.

## Example

Consider the following [repro](https://sharplab.io/#v2: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) for determining where to publish a NuGet package.