Improve location information for Objective-C category declarations. We

previously only had a single location (the @ in @interface); now we
know where the @ is (for the start of the declaration), where the
class name is (that's the normal "location" now for diagnostics), and
where the category name is. Also, eliminated the redundant "end"
location, since ObjCContainerDecl already has better @end information.

The only XFAIL'd test is temporary; will un-XFAIL-it once I've taught
CIndex how to use the new locations.

llvm-svn: 93639