Under -fms-extensions, only inject a friend tag name when we didn't see a tag with that name in an enclosing scope.

r177473 made us correctly consider only those declarations in the
enclosing namespace scope when looking for a friend declaration. Under
ms-extensions mode, where we do some level of friend injection, this
meant that we were introducing a new tag type into a different scope
than what Microsoft actually does. Address this by only doing the
friend injection when we didn't see any tag with that name in any
outer scope. Fixes <rdar://problem/14250378>.