rename Decl::CompatibleAlias -> ObjCCompatibleAlias.

Fix objc ivar lookup.  Ivar lookup should occur between lookup
of method-local values and lookup of globals.  Emulate this with
some logic in the handling of Sema::ActOnIdentifierExpr.

Two todo's left:
 1) sema shouldn't turn a bare reference to an ivar into "self->ivar"
    in the AST.  This is a hack.
 2) The new ScopedDecl::isDefinedOutsideFunctionOrMethod method does
    not correctly handle typedefs and enum constants yet.

llvm-svn: 48972