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).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224486 91177308-0d34-0410-b5e6-96231b3b80d8