Add an IR Verifier check for orphaned DICompileUnits.

A DICompileUnit that is not listed in llvm.dbg.cu will cause assertion
failures and/or crashes in the backend. The Verifier should reject this.

rdar://problem/25369499

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@264657 91177308-0d34-0410-b5e6-96231b3b80d8