Simplify a bit.

This follows from computeKeyFunction having:

  // Template instantiations don't have key functions,see Itanium C++ ABI 5.2.6.
  // Same behavior as GCC.
  TemplateSpecializationKind TSK = RD->getTemplateSpecializationKind();
  if (TSK == TSK_ImplicitInstantiation ||
      TSK == TSK_ExplicitInstantiationDefinition)
    return 0;