Pare back r164351 somewhat. The problem that change was addressing was that we
don't serialize a lookup map for the translation unit outside C++ mode, so we
can't tell when lookup within the TU needs to look within modules. Only apply
the fix outside C++ mode, and only to the translation unit.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178706 91177308-0d34-0410-b5e6-96231b3b80d8