[dsymutil] Fix handling of cross-cu forward references.

[dsymutil] Fix handling of cross-cu forward references.

We recorded the forward references in the CU that holds the referenced
DIE, but this is wrong as those will get resoled *after* the CU that
holds the reference. Record the references in their originating CU along
with a pointer to the remote CU to be able to compute the fixed up
offset at the right time.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232193 91177308-0d34-0410-b5e6-96231b3b80d8