Modules: Fix a minor performance bug from r293393

Oops... r293393 started calling ReadSignature in
ModuleManager::addModule even when there was no ExpectedSignature.

Whether or not this would have a measurable performance impact (I
spotted this by inspection, and ReadSignature should be fairly fast), we
might as well get what we can.  Add an extra check against
ExpectedSignature to avoid the hit.

llvm-svn: 293415