Don't replace an existing decl in the scope chains with its
local-extern redeclaration;  type refinements, default arguments,
etc. must all be locally scoped.

rdar://13535367

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179430 91177308-0d34-0410-b5e6-96231b3b80d8