Support RelatedInformation for analysis Diagnostic.

[ldez]

Closes #1189

RelatedInformation is an experimental field from analysis Diagnostic structure.

https://github.com/golang/tools/blob/b4639ccb830b1c9602f1c96eba624bbbe20650ea/go/analysis/diagnostic.go#L30-L31

The implementation adds this information as a new issue.

According to my research, only Staticcheck uses this field.

Example

package example

func fn(x *byte) {
	println(*x)
	if x != nil {
		return
	}
}

Before:

$ golangci-lint run
main.go:3:6: `fn` is unused (deadcode)
func fn(x *byte) {
     ^
main.go:4:10: SA5011: possible nil pointer dereference (staticcheck)
        println(*x)
                ^

After:

$ ./golangci-lint run
main.go:3:6: `fn` is unused (deadcode)
func fn(x *byte) {
     ^
main.go:4:10: SA5011: possible nil pointer dereference (staticcheck)
        println(*x)
                ^
main.go:5:5: SA5011(related information): this check suggests that the pointer can be nil (staticcheck)
        if x != nil {
           ^