[ELF] Replace SymbolTable::forEachSymbol with iterator_range symbols()

D62381 introduced forEachSymbol(). It seems that many call sites cannot
be parallelized because the body shared some states. Replace
forEachSymbol with iterator_range<filter_iterator<...>> symbols() to
simplify code and improve debuggability (std::function calls take some
frames).

It also allows us to use early return to simplify code added in D69650.

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D70505