* [dmd/parse] Refactor enum declaration
Tip: set ignore whitespace for diff.
Remove prevTOK.
Only use token loop for attributes.
"expected identifier after type" introduced in previous pull replaced
with "no identifier for declarator" for consistency.
Non-@ token "not a valid attribute" error replaced with "expected identifier".
"type only allowed if anonymous enum and no enum type" error was never
hit, this triggered "not a valid attribute" error instead.
No need to check `terror` as type will be null anyway.
"assignment must be preceded by an identifier" has been superceded by
other errors.
Make "enum declaration is invalid" error more informative.
* Add test for lone attribute
* Detect typed member with named enum for better error
* Add test for type error
* Avoid infinite loop
* Move 2 new tests to separate file; test `T @a b`
