[ELF] Use SmallVector for SharedFile and simplify parseVerdefs

SHT_GNU_verdef is typically small, so it's unnecessary to reserve the vector.

While here, fix a hypothetical issue when SHT_GNU_verdef has non-increasing
version indexes, which don't happen with GNU ld, gold, ld.lld's output.

My x86-64 lld executable is 256 bytes smaller.