Revert "DebugInfo: Assume all subprogram DIEs have been created before any abstract subprograms are constructed."

This reverts commit r209178.

This seems to be asserting in an LTO build on some internal Apple
buildbots. No upstream reproduction (and I don't have an LLVM-aware gold
built right now to reproduce it personally) but it's a small patch & the
failure's semi-plausible so I'm going to revert first while I try to
reproduce this.