[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