Save out a correct lookup table if a lookup table entry is stale (it contains
an out-of-date external decls list). This happens if we declare some names,
force the lookup table for the decl context to be built, import a module that
adds more decls for the name, then write out our module without looking up the
name.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204694 91177308-0d34-0410-b5e6-96231b3b80d8