[ELF] Simplify SectionTable::getSymbolTableIndex. No functionality change.

MSVC 2012 raises an error in the lambda passed to vector::find_if, while it
seems valid code. Rewrote without high-order functions.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@193975 91177308-0d34-0410-b5e6-96231b3b80d8