staticcheck: add check for modifying return variables within deferred function/statement

[iJustErikk]

I'd like to propose a check for modifying return variables within a deferred function or statement. In the case of errors, they end up getting swallowed, so this can be quite an interesting foot-gun.

Here is a small program that should trigger this proposed lint:

package main

import "fmt"

func test() int {
  var res int
  defer func() {
    res += 1
  }()
  return res
}

func main() {
	fmt.Printf("%d", test())
}

This should not be triggered by setting named returns.

This should also work in the unholy cases of redeclaring the return variables in the deferred functions or with nested deferred functions.

[iJustErikk]

I can take this up some weekend from now if we agree on the behavior

[dominikh]

This seems like a more general case of #590 and it is non-trivial to implement correctly, especially with our current representation of closures and defer statements, which considers all variables that are closed over as escaping to the heap, which means we can't rely on SSA for tracking their def-use chains.