Keep track of all declarations of an Objective-C class (both forward

Keep track of all declarations of an Objective-C class (both forward
declarations and definitions) as ObjCInterfaceDecls within the same
redeclaration chain. This new representation matches what we do for
C/C++ variables/functions/classes/templates/etc., and makes it
possible to answer the query "where are all of the declarations of
this class?"

llvm-svn: 146679