C++ modules: Don't call DeclContext::lookup when half-way through deserializing
decls. That can reenter deserialization and explode horribly by trying to merge
a declaration that we've not got very far through deserializing yet.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186236 91177308-0d34-0410-b5e6-96231b3b80d8