[ELF] Keep local symbols when both --emit-relocs and --discard-all are specified

This fixes a bug as exposed by D77807.

Add tests for {--emit-relocs,-r} x {--discard-locals,--discard-all}. They add coverage for previously undertested cases:

* STT_SECTION associated to GCed sections (`gc`)
* STT_SECTION associated to retained sections (`text`)
* STT_SECTION associated to non-SHF_ALLOC sections (`.comment`)
* STB_LOCAL in GCed sections (`unused_gc`)

Reviewed By: grimar, ikudrin

Differential Revision: https://reviews.llvm.org/D78389