Add -Wpartial-availability.

This warns when using decls that are not available on all deployment targets.
For example, a call to

  - (void)ppartialMethod __attribute__((availability(macosx,introduced=10.8)));

will warn if -mmacosx-version-min is set to less than 10.8.

To silence the warning, one has to explicitly redeclare the method like so:

  @interface Whatever(MountainLionAPI)
  - (void)ppartialMethod;
  @end

This way, one cannot accidentally call a function that isn't available
everywhere.  Having to add the redeclaration will hopefully remind the user
to add an explicit respondsToSelector: call as well.

Some projects build against old SDKs to get this effect, but building against
old SDKs suppresses some bug fixes -- see http://crbug.com/463171 for examples.
The hope is that SDK headers are annotated well enough with availability
attributes that new SDK + this warning offers the same amount of protection
as using an old SDK.

llvm-svn: 232750