The code in sw/source/core/fields/reffld.cxx had turned into whack-a-mole:  MSVC
needed the unreachable default:assert(false); to fall through to init some vars
to avoid C4701 ("potentially uninitialized local variable").  But Clang (with
!NDEBUG, and an implementation of assert marked as noreturn) reported that as
"fallthrough annotation in unreachable code", requiring the #ifdef NDEBUG guards
to be added.  And now clang-cl (also with !NDEBUG, but with the MSVC
implementation of assert apparently not marked as noreturn) complained about
"unannotated fall-through between switch labels", which wold require the #ifdef
NDEBUG guards to be removed again.

O3TL_UNREACHABLE (inspired by LLVM's llvm_unreachable and underyling
LLVM_BUILTIN_UNREACHABLE) hopefully improves on that for all compilers involved.
For non-MSVC (i.e., Clang or GCC) it assumes that __builtin_unreachable is
supported (which it is at least for GCC 7, which is our current baseline per
README.md).

Change-Id: I4474f95664d97c3d0e506bab90f80d5ab252ec6e
Reviewed-on: https://gerrit.libreoffice.org/67232
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>