Upgrade "'X' is unavailable" from a warning to an error. This matches GCC's behavior. Note that

GCC emits a warning instead of an error when using an unavailable Objective-C protocol, so now
Clang's behavior is more strict in this case, but more consistent.  We will need to see how much
this fires on real code and determine whether this case should be downgraded to a warning.

Fixes <rdar://problem/8213093>.

llvm-svn: 109033