Produce an expression tree for method groups.

[bbarry]

Suggesting this instead of commenting on #2545 because this shouldn't require any additional commitments from teams outside of the language team and should be completely compatible with existing expression tree usages.

Currently you can use method groups in place of lambda calls:

var allowed = users.Where(IsAllowed);

and it works for calls expecting a compatible delegate type.

It would be nice if you could do the same for locations where the compatible expression type is used and:

var allowed = users.Where(IsAllowed);

lowered to:

var allowed = users.Where(p => IsAllowed(p));

and then further lowered as this expression is today.