DI: Clarify isUnsignedDIType(), NFC

Refactor `isUnsignedDIType()` to deal with `DICompositeType` explicitly.
Since `DW_TAG_subroutine_type` isn't handled here (the assertions about
tags rule it out), this allows strengthening the `dyn_cast` to
`DIDerivedType`.

Besides making the code clearer, this it removes a use of
`DIDerivedTypeBase`.

llvm-svn: 243148