[NFC][unwind] Improve error message when a type has more than one RTTIs

The "dynamic_cast error 2" error can apparently happen when the same
type (with RTTI) is defined in more than one translation unit, and
those translation units are linked together. This is technically an
ODR violation, but making the error message more obvious is still
helpful.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@344052 91177308-0d34-0410-b5e6-96231b3b80d8