[DwarfDebug][NFC] Factor out 'isInitialized' logic

The class 'DbgVariable' can be in one of three states, and the "is any of them
initialization" logic for them is repeated in a couple of places. We may want to
expand this class in the future; as such, we factor out this common logic so
that it is easier to modify.

Differential Revision: https://reviews.llvm.org/D158438