[ELF] Change Symbol::verdefIndex from uint32_t to uint16_t

The SHT_GNU_version index is 16-bit, so the 32-bit value is a waste.
Technically non-default version index 0x7fff uses version index 0xffff,
but it is impossible in practice.

This change decreases sizeof(SymbolUnion) from 80 to 72 on ELF64 platforms.
Memory usage decreases by 1% when linking a large executable.