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`.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243159 91177308-0d34-0410-b5e6-96231b3b80d8