DI: Simplify DebugInfoFinder::processType(), NFC

Handle `DISubroutineType` up-front rather than as part of a branch for
`DICompositeTypeBase`.  The only shared code path was looking through
the base type, but `DISubroutineType` can never have a base type.

This also removes the last use of `DICompositeTypeBase`, since we can
strengthen the cast to `DICompositeType`.

llvm-svn: 243159