[yaml2obj] - Convert `ELFState<ELFT>::addSymbols` method to `toELFSymbols` helper. NFCI.

ELFState<ELFT>::addSymbols method looks a bit strange.
User code have to create the destination symbols vector outside,
add a null symbol and then pass it to addSymbols when it seems
the more natural logic is to isolate all work with symbols inside some
function, build the list right there and return it.

Differential revision: https://reviews.llvm.org/D63493

llvm-svn: 363930