implement whilelist for switch

[kolyshkin]

The following legitimate comparison using switch statement

func SwitchOnUnixErrors() {
       err := unix.Rmdir("somepath")
       switch err {
       case unix.ENOENT:
               return
       case unix.EPERM:
               return
       }
       fmt.Println(err)
}

results in a warning:

switch on an error will fail on wrapped errors. Use errors.Is to check for specific errors

Apparently, whitelist of allowed direct comparisons, as implemented in allowed.go, is not used when analysing switch statements.

[kolyshkin]

Another example:

func SwitchIoEOF(r io.Reader) {
        var buf [4096]byte
        _, err := r.Read(buf[:])
        switch err {
        case nil:
                return
        case io.EOF:
                return
        }
        fmt.Println(err)
}

[polyfloyd]

Yes, that should be a valid comparison. The allowlist must apply to switch statements too