Handles the cases where old __va_list_tag is coming from a module and the new
is not, needing an update record.

Fixes https://llvm.org/bugs/show_bug.cgi?id=27890

Patch by Cristina Cristescu, Richard Smith and me.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@273159 91177308-0d34-0410-b5e6-96231b3b80d8