checker: fix immutable to mutable reference

[felipensp]

Fix #22653

Fixes cases like this:

fn main() {
	a := ['foo', 'bar']
	mut b := []string{}
	b = if true { a } else { a } 	
	b[0] = 'bar'
	dump(a) // ['bar', 'bar']
}

_{Huly®: V_0.6-21112}

[felipensp]

What do you think @medvednikov @spytheman ?

[felipensp]

The failing CI here is really about breakage related to more restricter compiler checks.

[spytheman]

(rebased over current master)

[spytheman]

I think that is a very good change, but it should be a notice for a few weeks/months, to avoid sudden breaking changes to existing projects, and allow them to fix the new notices more gradually. It will also solve the CI issues here, and allow for running regression tests with .github/workflows/compare_pr_to_master.v (which needs to compile master and the branch with different versions of the compiler, which is prevented if one of the versions has stricter errors for the older codebase).