Tweak -Wdeprecated-objc-pointer-introspection to have a subgroup for results...

Tweak -Wdeprecated-objc-pointer-introspection to have a subgroup for results of using -performSelectorXXX.

-performSelector: and friends return a value that is boxed as an Objective-C
pointer.  Sometimes it is an Objective-C pointer, sometimes it isn't.
Some clients may wish to silence this warning based on calling
this method.

Fixes <rdar://problem/14147304>

llvm-svn: 184789