loopvarcapture: do not flag defer within local closure

[renatolabs]

Previously, handling of defer statements in the loopvarcapture
linter was naive: whenever a defer statement in the body of a loop
referenced a loop variable, the linter would flag it as an invalid
reference. However, that can be overly restrictive, as a relatively
common idiom is to create literal functions and immediately call them
so as to take advantage of defer semantics, as in the example below:

for _, n := range numbers {
    // ...
    func() {
           // ...
           defer func() { doSomewithing(n) }() // always safe
           // ...
    }()
}

The above reference is valid because it is guaranteed to be called
with the correct value for the loop variable.

A similar scenario occurs when a closure is assigned to a local
variable for use within the loop:

for _, n := range numbers {
    // ...
    helper := func() {
           // ...
           defer func() { doSomething(n) }()
           // ...
    }
    // ...
    helper() // always safe
}

In the snippet above, calling the helper function is also always
safe because the defer statement is scoped to the closure containing
it. However, it is still not safe to call the helper function within
a Go routine.

This commit updates the loopvarcapture linter to recognize when a
defer statement is safe because it is contained in a local
closure. The two cases illustrated above will no longer be flagged,
allowing for that idiom to be used freely.

Release note: None.

[cockroach-teamcity]

This change is 

[dhartunian]

I can confirm that this fixes the linter issue with #79663. Thanks!

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @srosenberg)

[dhartunian]

friendly ping! can we merge this so I can unblock my PR? the test case looks straightforward to me.

[srosenberg]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @renatolabs)

[renatolabs]

bors r=srosenberg,dhartunian

Thanks!

[craig]

Build succeeded:

• GitHub CI (Cockroach)