[REG2.066a] Issue 12924 - deprecated("foo"); and deprecated; should not compile

https://issues.dlang.org/show_bug.cgi?id=12924

'DeclarationBlock' that follows 'Attribute' should be parsed by `parseBlock`.

`PrefixAttribute` is used to check redundant/conflicting attributes over the mutual call between `parseDeclDefs` and `parseBlock`.