DI: Make getOrCreateLimitedType() return DICompositeType, NFC

Change `getOrCreateLimitedType()` to return a `DICompositeType` and
remove the casts from its callers.  Inside, I've strengthened a `cast`
from `DICompositeTypeBase`, but the casts in the callers already prove
that this is safe.  There should be no functionality change here.

llvm-svn: 243155