Convert all structs using inheritance into classes
Merge pull request #2033 from yebblies/structtoclass

[DDMD] Convert all structs using inheritance into classes
enum PROT -> PROT
enum TOK -> TOK
enum BUILTIN -> BUILTIN
enum PURE -> PURE
enum PASS -> PASS
enum Semantic -> Semantic
enum AliasThisRec -> AliasThisRec
enum TRUST -> TRUST
enum RET -> RET
enum LINK -> LINK
enum PREC -> PREC
enum Sizeok -> Sizeok
enum FLAGS -> FLAGS
enum STATE -> STATE
Merge pull request #2035 from yebblies/enumfix

[DDMD] Replace uses of `enum XXX` as a type with `XXX`.