elsif and else attribute

[dalance]

#1349 will add the way to judge define's exclusivity.
So the way to express exclusivity becomes important.

For example, the following code is multi-drive error, because FEATURE_A, FEATURE_B and FEATURE_C are not exclusive.

#[ifdef(FEATURE_A)]
assign a = 1;
#[ifdef(FEATURE_B)]
assign a = 2;
#[ifdef(FEATURE_C)]
assign a = 3;

The following code can express exclusivity.
In this case, each assign 's define context is exclusive, and not multi-drive error.
But this is too complicated.

#[ifdef(FEATURE_A)]
assign a = 1;
#[ifndef(FEATURE_A)]
#[ifdef(FEATURE_B)]
assign a = 2;
#[ifndef(FEATURE_A)]
#[ifndef(FEATURE_B)]
#[ifdef(FEATURE_C)]
assign a = 3;

So elsif and else is useful like below:

#[ifdef(FEATURE_A)]
assign a = 1;
#[elsif(FEATURE_B)]
assign a = 2;
#[elsif(FEATURE_C)]
assign a = 3;

elsif and else have some ambiguity.

#[ifdef(FEATURE_A)]
assign a = 1;

let x: logic = 1; // this statement is not in both `ifdef` and `elsif`.

#[elsif(FEATURE_B)]
assign a = 2;

Such case should be treated as error.