Reland "[PDB] Defer relocating .debug$S until commit time and parallelize it"

This reverts commit 5b7aef6e and relands
6529d7c5.

The ASan error was debugged and determined to be the fault of an invalid
object file input in our test suite, which was fixed by my last change.
LLD's project policy is that it assumes input objects are valid, so I
have added a comment about this assumption to the relocation bounds
check.