[LTO] Implement -disable-verify, which disables bitcode verification.

So, there are some cases when the IR Linker produces a broken
module (which doesn't pass the verifier) and we end up asserting
inside the verifier. I think it's always a bug producing a module
which does not pass the verifier but there are some cases in which
people can live with the broken module (e.g. if only DebugInfo
metadata are broken). The gold plugin has something similar.

This commit is motivated by a situation I found in the
wild. It seems that somebody else discovered it independently
and reported in PR24923.

llvm-svn: 265258