[ELF] Make --trace-symbol track preempted shared definitions
Note, we still name a preempted SharedSymbol "shared definition",
instead of "reference" as printed by GNU ld. This difference should not matter.
// GNU ld ld.bfd: t: definition of f@v1 ld.bfd: t.so: reference to f@v1
Reviewed By: psmith
Differential Revision: https://reviews.llvm.org/D80143