access to the members of the Objective-C self object.

The approach we take is to generate the method as a
@category on top of the self object, and to pass the
"self" pointer to it.  (_cmd is currently NULL.)

Most changes are in ClangExpressionDeclMap, but the
change that adds support to the ABIs to pass _cmd
touches a fair amount of code.

llvm-svn: 121722