Change CGObjCRuntime methods to take appropriate clang Decls.

Change CGObjCRuntime methods to take appropriate clang Decls.
 - This is in prep for implementation class support for the NeXT
   runtime, for which the existing methods don't provide enough
   information (and additionally make too many assumptions about how
   things should be emitted).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@54824 91177308-0d34-0410-b5e6-96231b3b80d8