DI: Remove unnecessary DICompositeTypeBase

Remove unnecessary and confusing common base class for `DICompositeType`
and `DISubroutineType`.

While at a high-level `DISubroutineType` is a sort of composite of other
types, it has no shared code paths, and its fields are completely
disjoint.  This relationship was left over from the old debug info
hierarchy.

llvm-svn: 243160