IR: Make DICompositeType mutators private

Make `DICompositeType` mutators private to prevent misuse.  All calls to
`setArrays()` and `setContainingType()` should go through
`DIBuilder::replaceArrays()` and `DIBuilder::replaceVTableHolder()`.

This is a follow-up to r224482 (now that clang has been updated in
r224483).