Relax current limitations for UDA syntax parsing.

[Issue 11677](https://d.puremagic.com/issues/show_bug.cgi?id=11677) - user defined attributes must be first
[Issue 11678](https://d.puremagic.com/issues/show_bug.cgi?id=11678) - user defined attributes cannot appear as postfixes
[Issue 11679](https://d.puremagic.com/issues/show_bug.cgi?id=11679) - user defined attributes not allowed for local auto declarations
[Issue 11680](https://d.puremagic.com/issues/show_bug.cgi?id=11680) - user defined attributes for type inference

Additionally, I added test cases for ddoc unittest feature to avoid silent breaking.