Support backward function definition

[taichi-ishitani]

Currently, backward function definition is not supported because wrong IT is generated for function call with such function.
Function prototype needs to be introduced for ease of generating IR.

module ModuleA (
    i_clk: input clock,
) {
    var a: logic;
    always_ff {
        if get_b() {
            a = '1;
        }
    }
    let b: logic = '1;
    function get_b() -> logic {
        return b;
    }
}

Wrong IR is generated for function call before defining the called function. Due to this, this error is happened.

I disccussed it with @dalance .
Backword function difinition is not supported for now because it is difficult to implement this feautre.

Originally posted by @taichi-ishitani in #2205

[taichi-ishitani]

Implement this by #2223.
Function prototype is not required.

[taichi-ishitani]

I discussed this with @dalance .

We agreed that it is too strict limitation to prohibit any function reference before definition at point of code readability.
And the limitation will be relaxled; Allow to call functions that have no reference to variables not defined at the time calling them.

Example:

module ModuleA {
    let a: u32 = 8;
    let b: u32 = func_a(); // OK

    function func_a() -> u32 {
        return a;
    }

    let c: u32 = func_b(); // Error; `d` is not defined yet.
    let d: u32 = 8;

    function func_b() -> u32 {
        return d;
    }
}

[taichi-ishitani]

For the above error case, Veryl compiler reports ReferringBeforeDefinition error to the variable d.
This error message is confusing.