At the begining of the module we can iterate through the functions to
see which SPs should have concrete DIEs. Then when we need to reference
a DIE for a SP we can decide if it's ok to create a concrete DIE or not.
This was previously landed in https://reviews.llvm.org/D112337 but was
reverted because of a memory bug. This patch fixes that bug by making
sure we don't reference abstract origins from another CU.