The main motivation is to further reduce the complexity of the class:
* There is no code that ever writes to $this->mSubstIDs. It's
effectively a constant.
* According to CodeSearch the getSubstIDs() method is not used
anywhere. It's @internal to the parser.
* I find it weird that the parser needs to call 2 factory methods to
do 1 thing.
* I still find it a good idea to keep the knowledge encapsulated in
the factory and not have the [ 'subst', 'safesubst' ] array in the
parser. That's why I propose the new method.
Change-Id: I5c147c75200c3c34a410d93a0328b56ea00a050f
10a828ba72