(added recently with d559c28e "expand out
IMPL_XTYPEPROVIDER_START macro"), for extra compatibility safety.  (At least
MSVC exports non-inline versions of such CPPUHELPER_DLLPUBLIC inline functions
from the cppuhelper DLL.)  The original getTypes (added back now with
<https://gerrit.libreoffice.org/#/c/68549/> "revert ABI change from 'expand out
IMPL_XTYPEPROVIDER_START macro'") had been a non-const member function ever
since at least b525a311 "initial import",
presumably by accident.  (Whether or not to return a reference is an orthogonal
issue.  With the newly added overload being LIBO_INTERNAL_ONLY, it is presumably
fine to have it return a reference for now and see whether that causes any
issues with lifetimes of temporary OTypeCollection instances.)

Change-Id: If6abcf53b46b972204598774fed7cdd34d78440b
Reviewed-on: https://gerrit.libreoffice.org/68637
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>