[modules] Fix false report of an ODR violation when merging friend
declarations. We can't expect to find them in the canonical definition
of the class, because that's not where they live.

This means we no longer reject real ODR violations with friend declarations,
but we weren't consistently doing so anyway.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216369 91177308-0d34-0410-b5e6-96231b3b80d8