Simplify logic for deconstruction variables in ExpressionVariableFinder

Per discussion in PR https://github.com/dotnet/roslyn/pull/14871

• Maybe we don't need MakeOutVariable and MakeDeconstructionVariable, instead one may suffice, by looking up the syntax tree.
• Maybe we don't need CollectVariablesFromDeconstruction in ExpressionVariableFinder, instead we just need a VisitVariableDesignation which looks up the syntax tree to determine if it is an out or a deconstruction variable.
• Then maybe we can re-use ExpressionVariableFinder instead of ForEachLoopBinder.CollectLocalsFromDeconstruction.