Use isExternalLinkage instead of hasExternalLinkage.

Having these not be the same makes an easy to misuse API. We should audit the
uses and probably rename to something like

foo->hasExternalLinkage():
  The c++ standard one. That is UniqueExternalLinkage or ExternalLinkage.

foo->hasUniqueExternalLinkage():
  Is UniqueExternalLinkage.

foo->hasCogeGenExternalLinkage():
  Is ExternalLinkage.