Re-apply "[JITLink][ORC] Rename MemDeallocPolicy to MemLifetime..." with fixes.

This reapplies 2cc64df0, which was reverted in
5379c46d due to bot failures.

The new patch contains fixes to ELFLinkGraphBuilder.h to better handle
non-SHT_ALLOC sections (these were being accidentally skipped in the previous
patch), and to skip SHT_NULL sections.