[modules] Do not report missing definitions of demoted constexpr variable templates.

This is a followup to regression introduced in r284284.

This should fix our libstdc++ modules builds.

https://reviews.llvm.org/D25678

Reviewed by Richard Smith!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@284577 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed