CodeGen: Don't emit linkage on thunks that aren't emitted because they're
vararg.
This can happen when we're trying to emit a thunk with available_externally
linkage with optimization enabled but bail because it doesn't make sense for
vararg functions.
[LLVM] PR18098.
This should fix clang "Broken module found, compilation aborted" errors when
building the qt4-based dvbcut port.
Reported by: se
MFC after: 3 days