ast, markused, checker: fix mark methods into used-list, when generics as receivers(fix #20509)

[shove70]

1. Fixed cgen: -skip-unused problem #20509
2. Add tests.

import time

fn foo[T](val T) string {
	str_value := val.format_rfc3339_nano()
	return str_value
}

fn main() {
	println(foo(time.Time{}))
}

v -skip-unused run a.v:

0000-00-00T00:00:00.000000000Z

[spytheman]

It is very weird that vab failed compilation on the CI to me. The code in the PR, can just protect more code to not be stripped, yet the failure indicates the opposite on first glance 🤔 .

[shove70]

It is very weird that vab failed compilation on the CI to me. The code in the PR, can just protect more code to not be stripped, yet the failure indicates the opposite on first glance 🤔 .

Yeah, it's a little weird. I'll do more research

[spytheman]

That comment should be deleted imho. markused does not modify receiver_concrete_type.

[spytheman]

Excellent work.

[shove70]

This indicates that the comments are still not clear enough, so I'll have to edit them :)