Bugfix :: Fix 'Type inference problem too complicated' for SRTP with "T:null and T:struct" dummy constraint

[T-Gro]

Fixes #18288.

The problem occured when inferring types and the solution contained a method defined like _: 'T when 'T: null and 'T: struct.
This problem is fixed.

I have also added consistency checks for pairs of typar constraints that are invalid - will wait in the issue comments for @gusty to see if reporting an error for this combination will break an essential trick without providing alternative.

I also realized that combination of delegate and comparison constraints is invalid as well, but was misusing a wrong error message -> it gets a new error message now.

With all the logic around typarTy and checks for their constraints having the following form:

            isTyparTy g ty
            && (destTyparTy g ty).Constraints
               |> List.exists (function
                   | TyparConstraint...._ -> true
                   | _ -> false)

,the code got convoluted with this boilerplate.
I extracted this logic into helper functions and applied them where they made sense:

ty |> IsTyparTyWithConstraint g _.Is....

[github-actions]

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