Audit __private_extern__ handling.

 - Exposed quite a few Sema issues and a CodeGen crash.

 - See FIXMEs in test case, and in SemaDecl.cpp (PR3983).

I'm skeptical that __private_extern__ should actually be a storage
class value. I think that __private_extern__ basically amounts to
  extern A __attribute__((visibility("hidden")))
and would be better off handled (a) as that, or (b) with an extra bit
in the VarDecl.

llvm-svn: 69020