Debugger does not enable watch for mutable fields

Compile an exe containing this:

````
let internal x = 10

let mutable internal y = x * 10

let mutable z = y + x

let a = 10

let printIt() =
//x <- x + 1
printfn $"{x} + {y} + {z}"

printIt()

printIt()
printIt()
printIt()
printIt()
printIt()
````

Step through it under the debugger.

Add x, y and z to the watch window:
observe y and z state: error CS0`103: the name 'y' does not exist in the current context.