ThinLTOBitcodeWriter: Do not follow operand edges of type GlobalValue when...

ThinLTOBitcodeWriter: Do not follow operand edges of type GlobalValue when looking for virtual functions.

Such edges may otherwise result in infinite recursion if a pointer to a vtable
is reachable from the vtable itself. This can happen in practice if a TU
defines the ABI types used to implement RTTI, and is itself compiled with RTTI.

Fixes PR32121.

llvm-svn: 296839