When deserializing a declaration, don't look for redeclarations if its
kind indicates that it can never be redeclared. Good for a 1% speedup,
and redeclaration searching drops off the profile.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173054 91177308-0d34-0410-b5e6-96231b3b80d8