See the updated insert-before.test for the effects: many synthetic
sections are SHF_ALLOC|SHF_WRITE. If they are discarded, we don't want
to propagate their flags to subsequent output section descriptions.
`getFirstInputSection(sec) == nullptr` can technically be merged into
`isDiscardable` but I'd like to postpone that as not sharing code may give more
refactoring opportunity.
Depends on D118529.
Reviewed By: peter.smith, bluca
Differential Revision: https://reviews.llvm.org/D118530
fbf2f66400