This is a large/messy diff that unifies the ObjC AST's with DeclContext.

- ObjCContainerDecl's (ObjCInterfaceDecl/ObjCCategoryDecl/ObjCProtocolDecl), ObjCCategoryImpl, & ObjCImplementation are all DeclContexts.
- ObjCMethodDecl is now a ScopedDecl (so it can play nicely with DeclContext).
- ObjCContainerDecl now does iteration/lookup using DeclContext infrastructure (no more linear search:-)
- Removed ASTContext argument to DeclContext::lookup(). It wasn't being used and complicated it's use from an ObjC AST perspective.
- Added Sema::ProcessPropertyDecl() and removed Sema::diagnosePropertySetterGetterMismatch().
- Simplified Sema::ActOnAtEnd() considerably. Still more work to do.
- Fixed an incorrect casting assumption in Sema::getCurFunctionOrMethodDecl(), now that ObjCMethodDecl is a ScopedDecl.
- Removed addPropertyMethods from ObjCInterfaceDecl/ObjCCategoryDecl/ObjCProtocolDecl.

This passes all the tests on my machine. Since many of the changes are central to the way ObjC finds it's methods, I expect some fallout (and there are still a handful of FIXME's). Nevertheless, this should be a step in the right direction.

llvm-svn: 61929