[IR/Verifier] Fix (yet another) crash.

We need to check that if we reference a retainedType from
DICompileUnit we're actually referencing a DICompositeType.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@265752 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed