Partial revert of r203179. The build bots are telling me that some out-of-tree builds are using redecls_begin()/redecls_end(), so adding some machinery back in to support them.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203182 91177308-0d34-0410-b5e6-96231b3b80d8