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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@137349 91177308-0d34-0410-b5e6-96231b3b80d8