[modules] When completing the redecl chain for an anonymous declaration in a
merged context, pull in the lexical decls in that context, since one of them
may complete the redecl chain.


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@216652 91177308-0d34-0410-b5e6-96231b3b80d8