SA4006: build tags and control flow

[dominikh]

The latest entry in our "build tags hate us" saga: SA4006.

func gen() int { return 42 }
func die() { panic("good bye") }

func Fn() {
	x := gen()
	println(x)
	x = gen()
	die()
	println(x)
}

we flag the second assignment to x here, because we recognize that control flow doesn't continue after die gets called. Things get problematic when die turns into feature that is only implemented on some OSs and panics unconditionally on others thanks to build tags. Now we flag an assignment that is only conditionally unused, depending on the build tags in use.