Issue 11396 & 12802 - Allow optional 'StorageClasses' for old/new alias syntax

[Issue 11396](https://issues.dlang.org/show_bug.cgi?id=11396) - Function alias declaration not valid according to spec
[Issue 12802](https://issues.dlang.org/show_bug.cgi?id=12802) - Allow optional 'StorageClasses' for new alias syntax