[C# diagnostics] - Expression-form context restrictions

[BillWagner]

Describe the issue or suggestion

Impacted diagnostics:

Code	Actual message text	What triggers it	Feature
CS8115	A throw expression is not allowed in this context.	A throw expression appears outside the contexts where C# permits throw expressions.	Throw expressions
CS8185	A declaration is not allowed in this context.	A declaration expression appears where the grammar/binder forbids it.	Expression variables
CS8188	An expression tree may not contain a throw-expression.	An expression tree lambda contains a throw expression.	Expression trees / throw expressions
CS8209	A value of type 'void' may not be assigned.	Source attempts to assign the result of a void expression.	Expression binding
CS8310	Operator '{0}' cannot be applied to operand '{1}'	null, default, or new is used with an operator that has no valid operand binding.	Default literal / typeless expressions
CS8312	Use of default literal is not valid in this context	The default literal appears without a target type or in another invalid context.	Default literal

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Associated WorkItem - 607281