code cleanup prior to optional interop improvements (#7276)

* add test cases we need to make work

* cleanup method call argument processing

Author: dsyme

src/fsharp/ConstraintSolver.fs

@@ -1931,7 +1931,7 @@ and SolveTypeIsNonNullableValueType (csenv: ConstraintSolverEnv) ndeep m2 trace
         | _ ->
             let underlyingTy = stripTyEqnsAndMeasureEqns g ty
             if isStructTy g underlyingTy then
-                if isAppTy g underlyingTy && tyconRefEq g g.system_Nullable_tcref (tcrefOfAppTy g underlyingTy) then
+                if isNullableTy g underlyingTy then
                     return! ErrorD (ConstraintSolverError(FSComp.SR.csTypeParameterCannotBeNullable(), m, m))
             else
                 return! ErrorD (ConstraintSolverError(FSComp.SR.csGenericConstructRequiresStructType(NicePrint.minimalStringOfType denv ty), m, m2))