This reapplies 2cc64df0bd6a802eab592dbc282463c3e4a4281c, which was reverted in 5379c46d490640bfa80283e00240b6f1006092b4 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.