DebugInfo: Delete old subclasses of DIType

Delete subclasses of (the already deleted) `DIType` in favour of
directly using pointers from the `Metadata` hierarchy.

While `DICompositeType` wraps `MDCompositeTypeBase` and `DIDerivedType`
wraps `MDDerivedTypeBase`, most uses of each really meant the more
specific `MDCompositeType` and `MDDerivedType`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235351 91177308-0d34-0410-b5e6-96231b3b80d8