Fix a typo in rL265762

I accidentally replaced `mayBeOverridden` with `!isInterposable`.
Remove the negation and add a test case that would've caught this.

Many thanks to HÃ¥kan Hjort for spotting this!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@266551 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed