Throw expression

Champion issue: https://github.com/dotnet/csharplang/issues/62

We extend the set of expression forms to include

antlr

throw_expression
    : 'throw' null_coalescing_expression
    ;

null_coalescing_expression
    : throw_expression
    ;

The type rules are as follows:

A throw_expression has no type.
A throw_expression is convertible to every type by an implicit conversion.

A throw expression throws the value produced by evaluating the null_coalescing_expression, which must denote a value of the class type System.Exception, of a class type that derives from System.Exception or of a type parameter type that has System.Exception (or a subclass thereof) as its effective base class. If evaluation of the expression produces null, a System.NullReferenceException is thrown instead.

The behavior at runtime of the evaluation of a throw expression is the same as specified for a throw statement (§12.10.6).

The flow-analysis rules are as follows:

For every variable v, v is definitely assigned before the null_coalescing_expression of a throw_expression iff it is definitely assigned before the throw_expression.
For every variable v, v is definitely assigned after throw_expression.

A throw expression is permitted in only the following syntactic contexts:

As the second or third operand of a ternary conditional operator ?:
As the second operand of a null coalescing operator ??
As the body of an expression-bodied lambda or method.