[ELF] Reword symMap/symVector comment. NFC

Having symVector makes iteration efficient and is actually more
efficient than using llvm::DenseMap<llvm::CachedHashStringRef, Symbol
*>, so the FIXME comment can be removed. Using an alternative
implementation ankerl/unordered_dense.h decreases link time for chromium
by 0.x% but I am unsure it justifies the extra header file.