DWARF: Avoid cross-CU references under Fission

DWARF: Avoid cross-CU references under Fission

Turns out that the Fission/Split DWARF package format (DWP) is currently
insufficient to handle cross-CU (ref_addr) references. So for now,
duplicate any debug info needed in these situations:
* inlined_subroutine's abstract_origin
* inlined variable's abstract_origin
* types

Keep the ref_addr behavior in general, including in the split DWARF
inline debug info that can be emitted into the object files for online
symbolication.
Keep a flag to use the old (ref_addr) behavior for testing ways of
addressing this limitation in the DWP tool (& for those not using DWP
packaging).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302858 91177308-0d34-0410-b5e6-96231b3b80d8