checker: rewrite comptime_if_cond() to support comptime if cond evaluate

[kbkpbot]

This PR:

1. Rewrite comptime_if_cond() in checker, fully support comptime if conditon evaluate to true/false.
2. Add comptime iter $for support for enum/method/method params/sumtype variants/attribute.
3. Fix issue $if in $for attribute, does not issue error when attr field not exisit #25099, $if with (same) test ||'ed seems to be silently ignored #24938.

for test vlib/v/gen/c/testdata/comp_if_unknown.vv

fn main() {
        mut x := 1
        $if glibc {
                x = 2
        } $else {
                x = 3
        }
        println('done')
}

I think the output should contain :

#if defined(__GLIBC__)
x = 2;
#else
#endif

instead of :

#if defined(__GLIBC__)
x = 2;
#else
x = 3;
#endif

[huly-for-github]

Connected to Huly®: V_0.6-23643

[spytheman]

Excellent work 🙇🏻‍♂️ @kbkpbot .
Thank you very much for working on this.

[spytheman]

This PR also has a positive effect on compiling small programs too: