Fix for "DICompileUnit not listed in llvm.dbg.cu" verification error after ...

Fix for "DICompileUnit not listed in llvm.dbg.cu" verification error after ...

...cloning a function from a different module

Currently when a function with debug info is cloned from a different module, the
cloned function may have hanging DICompileUnits, so that the module with the
cloned function fails debug info verification.

The proposed fix inserts all DICompileUnits reachable from the cloned function
to "llvm.dbg.cu" metadata operands of the cloned function module.

Reviewed By: aprantl, efriedma

Differential Revision: https://reviews.llvm.org/D66510

Patch by Oleg Pliss (Oleg.Pliss@azul.com)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@370265 91177308-0d34-0410-b5e6-96231b3b80d8