Should not permit default literals in relational patterns

[alrz]

Version Used: master

Steps to Reproduce:

    public void M(byte i) {
        _ = i is > default; // compiles
        _ = i is default; // error
    } 

Expected Behavior:

A default literal 'default' is not valid as a pattern. Use another literal (e.g. '0' or 'null') as appropriate. To match everything, use a discard pattern '_'

Actual Behavior: No error

[jaredpar]

@gafter can u take a look at this?