Encapsulate the Objective-C id/Class/SEL "redefinition" types in

ASTContext with accessors/mutators. The only functional change is that
the AST writer won't bother writing the id/Class/SEL redefinition type
if it hasn't been explicitly set; previously, it ended up being
written as a synonym for the built-in id/Class/SEL.

llvm-svn: 137349