simplify "no delete" logic in SvRefBase

by converting the bit munging to use bitfields.
Remove unused return values.
Add asserts to check that AddRef() is not called after the object
is deleted.
Fix the code in SfxObjectShell to not call AddRef() after
SfxObjectShell is deleted.

Change-Id: I3a3565a0bc45fc9d1d086222265ab8b8175818a7
basic: -Werror=sign-compare

Change-Id: I7c776faa74a61b0d263c3b446972da9461b3d4f6